Glad to help. I've got nearly 20 years experience living on battery power, and enjoy helping others to avoid the mistakes I made.
The intended electrical loads are very helpful into how much battery she can get away with, and for how long the battery needs to be viable.
Number one, is the Laptops usage. This is the single biggest electrical consumer in my rig. If you look at the AC/DC power brick it will say something like output 19.5vDC and 4.5 amps. volts times amperage equals wattage.
So in this example the acdc power supply Is capable of supplying 87.75 watts. This however does not mean the laptop when it is on, wil be consuming that much. I have a 90 watt power supply, but typoing this the actual wattage consumed jumps between 26 and 42 watts, say an average of 35 watts, just online with one window open. insert a dvd and this is 55 to 60 watts. Streaming 1080i video, is 55 to 70 watts. But this is a 2010 dell laptop running windows 7. What yoyur daughter's and her friends laptops actually consume in use depends on teh laptop and the task it is doing, and the state of charge of its internal battery.
Actual amperage/ wattage consumed is great data to have. On Ac power a tool called a Kill a watt is abou 25$ and will give a wattage figure. But batteries are not rated in watt hours, but in amp hours. So one must do a little math, say the battery is at 12.4v amnd the load is 35 watts, how many amps is this? 35 watts divided by 12.4 watts is 2.8 amps of load.
Ok so a group 27 marine battery is about 100 amps hours. This means when it is new, and helathy, and fully charged, it can power a 5 amp load for 20 hours before voltage falls to 10.5v, which is considered 100% discharged. However this does not mean it can power a 20 amp load for 5 hours. The larger the load, the less actual capacity the battery has. This is called the Peukert effect.
Often newbs to living on battery power have no Idea how much battery they are using, how much is left, and actual amp hour counting meters, the good ones, are expensive, require some modification of the vehicles ground cables to pass through a shunt, and then need to be programmed correctly, zeroed when it is know the battery is truly full, and regularly reset when the battery is known to be full and the amp hour counter is not agreeing with the other tools which say it is.
Obviously not something for a couple week long road trip. So what to do, is the simple voltmeter. Get one which reads at least a hundredth of a volt. 12.34v is much more informative than 12.3. Voltage can be extremely misleading though. But parked, discharging powering only 2 laptops and a light or 2, if the battery reads 11.93v that can be a sign one is in the 50% charged range. If they turn off the laptops and in 5 minutes voltage rebounds to 12.4v, then it is more a sign the battery is in the 60 to 65% charged range. The bigger the electrical load the more the battery voltage will fall. How much it rebounds and how quickly it rebounds when the loads are removed, hep one to estimate the state of charge of the battery.
In general the 'rule' which I skoff at a bit, is to not let the battery go below 50% charged. I do not fear this threshold, as long as the nect day I can insure the battery is going to get fully charged or nearly so, but I do not like to really go below 25% charged as that is like a kick in the balls to it. It is not going to be happy especially if the next day it is not recharged to a true 100% state of charge.
One other factor is how many partial state of charge cycles have accumulated inbetween true full charges. Say one discharges to 50% and only gets to 87% before the next discharge cycle begines, that 100 amp hour battery will not have 87 amps hours to give, but each day will walk down a bit farther in capacity and take even longer to reach the same state of charge when recharged. After a week the battery will be at a fraction of its ability and is craving a true full charge, and when it was new and well depleted this might take 6.5 hours, but when PSOC cycled for a week to 10 days this will take 10 to 15 hours, or more. if it does not get this then it is likely that 100Ah battery now only will have 90Ah total available, and recharge a bit slower forever after, which then in turn insures reaching lower states of charge when recharging, which then accellerates capacity loss.
So they might start out the trip with more than enough battery capacity, but by week 6 they can only run their laptops or half as long before voltage drops below 11.9 and does not rebound above 12.0 when they shut it off.
Progressive capacity loss from chronic undercharging in partial state of charge deep cycle duty is a primary battery killer, and the only real way to stave it off is to insure regular true full charges, or at least getting close to that ideal.
But an alternatoive strategy is to simply accept the battery had been kicked in the balls too much to keep being useful, and return it to walmart where hopefully they just hand you a new one and one resets the battery clock.
Keeping a battery super healthy can be taken to ridiculous extremes, and for a few week cross country trip, one can view it as disposable. It is no big deal, unless one is way out in the sticks far from any outlet where the battery can be replaced.
Another factor is flooded batteries offgass when charging. They have to offgass to reach full charge. This electrolysis releases oxygen and hydrogen, and these gasses taks a sulfuric acid mist along with them, which smells like rotten eggs. Sulfuric acid mist is a known carcinogen. The tolernace of any specific human to the smell varies widely, but the mist can also destory ones ability to smell it.
One can read reports all over the web about how person X has been sleeping atop charging batteries for x years without any issue, and person Y breaks out in hives anfd coughing fits in the same situation.
A lot of people avoid the offgassing with an AGM battery in the passenger compartment. I have an underbody battery box, that for years held 2 flooded batteries, but even with a sealed hatch I could still smell them when they were charging at higher voltages/states of charge. I despise the smell.
If you put a battery inside the van body, please make sure it is securely attached to the floor. If it is a flooded battery it is wise to insure there is plenty of ventilation when the battery is charging quickly or at higher states of charge.
The Hair drier on an inverter is a battery murderer. A 1600 watt hair drier, trough an inverter, will draw 1800 watts minimum from the battery. A single healthy group 27 flooded fully charged marine battery, under a 1800 watt load, will instantly fall into the mid to low 11 volt range. lets say 11.4. 1800 divided by 11.4, is a 158 amp load.
This peukert calculator says a 100 amp hour battery can power a 158 amp load for 0.15 hours before falling to 50% charged. Under a 158 amp load the 100Ah battery is no longer 100Ah, but only 36.3 amp hours of capacity. That is also with a likely highly generous peukert number of 1.2. Flooded batteries dont often state their peukert number, and the best AGMS like Odyssey use a 1.15. I suspect flooded marine batteries are more like 1.22 to 1.30 when they are still healthy. Anyway the following calculator can give a good idea how long a healthy fully charged battery can power a load. Keep in mind that it always is assuming a healthy fully charged battery, and that almost never happens in real life.http://www.csgnetwork.com/batterylifecalc.html
It is easier to use less electricity than add more charging sources. If your dodge is like mine all the light fixtures use a incandescent bulb with a t10 wedge base bulb. but even if it uses an Ba15s bulb, like an 1156 used in your reverse lamps,. all of these have drop in replacement LEDS, which will be brighter and use significantly less energy than incandescent. Switching from incandescent bulbs to LED lighting is pretty much a no brainer. Some LEDs can be so obscenely bright that they do not use all that much less energy though. Anyway remove some interior bulbs, if you have not already, and do some searches. there are a couple caveats with leds, in that incandescent does not care which side is + and which is -, but many LEDs do and the fixture might need its wires swapped if an 1156 bulb base, or with a t10 just flip it 180 degrees.
Another way to save energy it to use a DC to DC laptop 'car adapter' to power the laptops, instead of using an inverter to power the AC/DC power brick the laptop came with. Check Ebay and amazon . Plug in laptop make and model and add 'car adapter'
these will come with a 12v ciggy plug connector. If the laptop's power brick cannot draw more than 60 watts these horrible connectors should be OK for a few week excursion before the4 plug melts internally and hopefully blows the fuse only. Ciggy plugs and receptacles are horrible electrical connections. Ubiquitous and convenient, but a problem waiting to happen, its only a matter of time and current. the more current the less time one has before the inevitable failure.
I Use Anderson powerpoles. This is a whole different topic but in short Anderson powerpoles come in 15/30 and 45 amp versions. All have the same plastic housings, a 45 will mate with a 15, but a 15 amp is for thinner wire gauges, the 45 amp versions use 10 to 12 gauge wire. the 15 and 30 amps versions one does not necessarily require the special crimping tool they sell, and a person good at soldering can skip the crimping of they want. However the 45 amp contacts are quite difficult even for an experienced crimper to crimp properly without the special crimp dies.
I pretty much exclusively use 45 amp powerpoles everywhere. and use them as charging inputs or 12vDC outputs.
They are not an inexpensive connector and are a bit fragile if stepped on.
XT60 connectors are much cheaper, but require good soldering skills.
There are OK versions of ciggy receptacles, but one is pretty much at the mercy of the laptop car adapter's provided ciggy plug.
Since the ciggy plugs are so problem proine when asked to pass more than 60 watts regularly, many peolpe decide instead to just use the original powerbrick plugged into an inverter. This certainly works, but is less efficient.
My DC to DC laptop adapter averages 8 to 12 watts less consumption than using the inverter to power the original power brick provided with the laptop. That is a significant savings, an average of 35 watts, compared to 47.
Generally the inverter is often seen as the solution to powering every household appliance by battery power, but in my opinion it is the single best way to quickly overdeplete the battery. I have one. I use it, as little as possible.
many 12v DC tvs exist. Mine is several years old, has a 13.3 inch flat screen, draws about 1.1 to 1.3 amps watching broadcast Tv stations and about 1.8 amps spinning a DVD. Swithcing on my laptops dc to dc power brick, watching certain tv stations, knocks those tv stations out. but so does the inverter powering the AC to DC power brick. I can turn on my inverter, have it power nothing( it consumes 0.61 amps turned on powering nothing), and actual tv channels 8 and 10, which are very strong in San Diego, disappear. The inverter might or mioght no be a factor in being able to acquire and watch certain TV stations, but its nice to not have to use the inverter to power the tv..
Home depot is carrying a thin frame 100 watt 12v solar panel for 75$. One of these in the center of the roof will keep the bateryt much happier and can take significant loads off the alternator. My 50/120 amp nippondenso alternator was 156 delivered, from Autozone. it might not be a drop in replacement in your 85. it will likely bolt in and align properly, but the wiring could be a bit different. I understand this can be intimidating. if you wish to upgrade the alternator, I can help.
looking at the photos, I am less sure, my alternator appears significantly thicker. Do you have a lot of room behind it?.. A dual V belt pulley can easily handle turning a 120 amp alternator, I only use one V belt though I have a dual V belt pulley. Perhaps you can have an autoparts store pull out the one designated for your 85 and one for an 89 and compare the mounting feet distance from tabletop when placed on the pulley face. Mines located on the passenger side just in front of the manifold.
i think its likely an old 60 amp alternator is not going to be happy having to regularly recharge a depleted house battery. Upgrading to a higher capacity alternator before they leave can save them a few days of headaches.
If your daughter is keen to learn some basics of living on Dc electricity, here are a few links that are well written: https://marinehowto.com/automotive-alternators-vs-deep-cycle-batteries/https://marinehowto.com/what-is-a-deep-cycle-battery/
He has plenty more helpful links, but the marine guys take things several steps further than really necessary in a Van. He states the proper and best way to do certain things, he does not say you can get away with this...by doing this.
The 'what is a deep cycle battery' link can be summed up as 'beware of the sticker'
Most marine batteries which say deep cycle, are no more durable than a starting battery in deep cycle duty, they just cost more, have less/no warranty but have threaded studs in addition to automotive posts for connections.
A couple week excursion a true deep cycle battery and a proper charging system to insure it can achieve 1000+ deep cycles, is not really required.
But it is entirely possible to kill a marine battery in a few weeks by using it hard nightly, and then chronically undercharging it.
I try to outline what Ideal would be, but then stress that one can spend ridiculous sums and spend way too much time truing to achieve it. its often much easier and less stressful to view the battery as more disposable. the gola then is to not be surprised when it is done, but to know the warning signs. Like voltage falling much more rapidly when not using it nearly as much, and a long extended recharge not positively affecting this.