I dont have any schematics of my own system, but there are some vanlife type forums out there with such info.
There are so many different ways to do things and thousands of products for battery isolation with engine off/ combination with it on, that the subject can be turned into a book few would ever read.
I recommend not cheaping out on the Fuse block/ buss bar, and these two can be considered the power distribution system from the second battery.
Proper wire terminations is paramount. proper crimping is not done with a crap tool and no experience, but this can also be taken to ridiculous extremes as well, far beyond what A van needs but minimum for a blue water yacht where a failed connection can mean an ugly death.
My own system uses 2 alternators feeding their own battery bank. I can switch the Starter to either battery bank, and I can power all the loads with either battery bank, but each alternator is dedicated to its own battery bank.
I've modified my externally regulated alternators to be controlled by a modified external adjustable voltage regulator, and choose the voltage via dials on my dash next to my voltmeters and ammeters.
I've 2 adjustable voltage power supplies, one 40 amp, the other 100 amp, for when I can plug into the grid to charge my batteries.
Keep in mind most everything everybody says about lead acid battery charging is insanely wrong. It takes no less than 3.5 hours to get an 80% charged healthy battery back to 100%, and this 3.5 hours assume a charging source capable of instantly bringing the battery to and holding the battery at 14.2 to 14.8v for that entire time.
When deep cycling lead acid batteries, it is important to not only recharge them to true full as often as possible, as soon as possible, some higher end AGM batteries need a minimum initial charge current in order to live an acceptable cycle life. Odyssey AGM specifies no less than 40 amps initial current on a 100 amp hour group 31 battery, and I can definitively say that feeding it less than this over and over, cycle after cycle is like kicking it in the balls, and saying now go run a marathon.
Achieving ideal charging for ideal battery life is likely far more money and labor intensive than simply replacing batteries more often. It can be taken to extremes, and I personally have issues determining 'good enough' for others, and will only outline what is ideal.
I've largely given up trying to inform on this topic, as there is just so much bad information out there and too many with little more than zero experience/knowledge, who are threatened by correct information blowing up what they have believed and re-repeated as gospel, since grandpa told them 40 years ago.