I don't camp in mine just use it for repairs ect so I only use one AGM deep cycle 12v battery connected to a 10amp converter charger I can plug in at home or to a generator. The converter charger I have is used for charging and maintaining batteries for trolling motors I picked up from a sporting store. It was on sale for $50 the actual RV ones run well over a $100. I also have the battery connected to the 12V aux pin in the 7pin trailer connector which charges the battery when my pickup is on(at least on the newer fords).
To do this do you have a diode or something so the 12 volt battery does not feed back to the truck battery or does that not matter? Also do you just have your AUX lead go to the battery and that charges it or do you need a charger from the lead to the battery?