If you can stick with 6 volts I would do that. It seems you get a few more AH for a similar sized battery. We had twin 6 volts with a total of 220 AH in our former travel trailer. But we couldn’t fit the same batteries in our new Arctic Fox TC so we went with two 12 volts and still have 200 AH. I can’t remember how each pair were wired but I think the 6 volts were wired in parallel and the 12 volts are in series – or maybe it’s the other way around! Anyway – lots of info available on the web.