Inverter draw from battery for residential fridge

My rig is a fifth wheel so I do not have a tongue (on the trailer...lol) I figure the truck is helping to keep the battery charged as I am driving. I am exploring other battery options such as 6 volts wired in series (not enough room for 4 of them to run them parallel and in series. With Lithium Ion, don't I need to get a different converter to charge them. Every time I think I understand this stuff, I come upon something that makes me confused.