Our first Class C had two 12V batteries. In Parallel, that doubles the amp hours available. We didn't need all that capacity since we were only traveling to sites with water and electricity.
What I found, though, is that two 12V's don't seem to play well together. One tends to decline a little and ruin the other. Threads here seem to bear that out from others' experiences. Two 6V's don't seem to have that issue. Problem with 6's is of course that both have to be functional since the coach needs 12V.
So... Given you have space for two batteries, if you need the capacity, check height clearance for 6V GC batteries and install a pair in series for 12V. But if all you need is for travel, chilling fridge at home, etc. I'd say get a good 12V DEEP CYCLE battery and save a few $.
As pointed out. Not Marine Start, not Dual Purpose, but full Deep Cycle, such as a Trolling Motor battery. I was in WallyWorld just yesterday and stopped at the batteries, looking at the GP 24's which is an automotive battery group, they also had specialty 24's. Marine Start, Marine Dual Purpose, and Deep Cycle. The Deep Cycle was the most expensive.
Personally, I like WallyWorld Batteries. They aren't the cheapest or the most expensive, but I've had good luck with their durability. Every bit as important to me, there are WallyWorld's everywhere, and many are 24-hour. It's great to be able to get one from a nearby source, any time, any day.