BFL - maybe I'm missing something, but why taking for recharge in the morning all batteries "but one", when he is barely using any electricity in daytime? He can take all for recharge if shower can be delayed until the evening. IMHO, in a setup like this shower is a luxury, and you don't need 12V pump to fill the kitchen pot from 5 gallon jar.
Another thing that I still don't understand, is how he is going to recharge them - from the truck alternator while driving an hour or two? Ok, maybe it's better than nothing...
And the last thing that I can't understand - why bother with any charging at all, if they only stay 5 days and don't use more than 20 AH a day. 2 Group 27 will give exactly 100 AH of usable energy. Some small solar won't hurt - and then why not installing it on the roof - but there is no need to haul heavy batteries back and forth from truck to trailer, or burn more fuel than you have to, to get some little charge.
Staying warm after sunset and before you are going to bed, can be a problem, alright, get a catalytic heater. Energy and propane savings will be significant, compared to furnace, and it is quiet. In the morning, as others noted, this small space gets warm as soon as you turn the gas stove on.
