Two 6 volt batteries wired in series would yield 12v. and the maximum available amp hours. A pair of 6 volt batteries would likely be heavier that than 12 volt batteries, but don't know how much more weight.
I run 2 group 27 12v. batteries hooked up in parallel on our TT when we're dry camping. I got 2 1/2 days on them with careful power management, including running the furnace as needed, which thankfully isn't needed that much down here in the winter..........
I did have to modify the battery box mounting to get the two battery boxes in. Both of them wouldn't fit sitting in the factory installed angle iron welded on the tongue.