I have no experience converting 120vAC fridges to be 12vDC fridges and am not quite sure how that is accomplished, other than powering the fridge from an inverter.
I know some of the guys use a thermostat which turns on the inverter just for the compressor then off again afterwards, which can save a lot of energy, depending on just how much energy the inverter consumes when the fridge compressor is not running.
The fact that 12vdc fridges are so dang expensive has turned many down the 'what is cheaper' route?
People that do full time and do the inverter dorm fridge route usually find their batteries lose capacity quickly as they do not have enough battery or recharging capacity. Many wish they had saved their time and effort and some more money for a 12v model instead.
There are some cheaper chest style plug in fridges available, but I can't speak to their function or reliability. One does not need to go the front loader dorm fridge route.
https://www.amazon.com/dp/B07DNNMY5...d_r=addb70e6-49a2-11e9-8448-5f8f8266119aDo beware of the common ubiquitous 12v powerport plug and receptacle. Most portable DC fridges will come with these but the wiring to and from them is too thin to support the long term passing of 35 to 70 watts. In addition to that the design of the ciggy plug will work its way loose. It is always best to bypass this plug on any device which needs to be reliable, and any device which can regularly ask for more than 40 watts. I know there are heavier duty ciggy plugs and receptacles, some of which claim 20 amp ratings(240 watts), but they are smoking crack, and 20 amps might be ok very intermittently, not consistently.
If one really wants to use 12v plugs, then at least wire the aftermarket quality receptacle with 10AWG right to the battery, fused at the battery for 30 amps. My 89 dodge wired the dash ciggy plug from fuse block with 18awg. The voltage drop on such a circuit will have the fridge low voltage protection kick in well before the battery is in any danger of being overdepleted.
A good semi budget friendly DC fridge, which uses a good compressor, the danfoss secop bd35f, is available from here:
https://www.truckfridge.com/all-models/This guy is a vendor for them and Vitrifrigo and has free shipping:
http://www.westyventures.com/parts.html#cart_restoreI now they are still pricey for just occassional weekend use, but the ones with both AC and DC an be plugged in at your home and used as a second fridge or freezer if that helps justify the cost.
If one decides just to do the inverter dorm fridge thing, expect to get about 2 days from a new healthy fully charged group31 marine battery before the low voltage alarm on the fridge starts screaming. Obviously ambient temperatures and how often the door is opened and for how long plays a big factor, as does the parasitic draw of the inverter when teh compressor is not actively running.
Be sure to do an extended and full recharge of the battery once the outing is over, by a plug in charger capable of no less than 10 amps. Alternator's voltage regulation is no where near adequate to fully charge a well depleted battrey, and even if the voltage regulation were ideal for this duty, it would take no less than 6.5 hours of driving. Since most voltage regulators revert to the mid to high 13's. The time to fully recharge a well depleted healthy battery is much much longer, and an older sulfated battery will likely never reach full charge at 13.8v, no matter how long it is plugged in and charging.
A lead acid battery ideally always wants to be fully charged. Anything less than this will have it lose capacity, and the lower and the longer it sits less than fully charged, the faster the capacity loss.