garyemunson,
Well Sir, I've done exactly what you're thinking of. Our coach, an '04 Itasca Horizon 36GD with the C-7 330HP CAT, came with (3) 12V "supposed" deep cycle batteries for house batteries. Well, for reasons not necessary to repeat here, I decided to change the (3) 12V for (4) 6V G/C batteries. It's very easy to wire them for this kind of service. I used the original cables from the batteries to the coach and made my own "link" cables that go between them.
We do "dry camp" on an often basis and, having that arrangement of batteries, sure works well for us in terms of duration. They do last quite a bit longer than the 12v units did, in the same given operational scenarios. You of course have to determine if you have the room for four of them. Speaking of that, if I were you, I wouldn't mess around with just changing to TWO 6 volters. If you're going to change, then do it with FOUR, not two. Get it done the right way, right off the bat. You'll thank yourself for it.
Scott
I'd post pics of the before and after the change but, on RV.net, it's a pain to do so.
Scott