I have two Interstate 6V batteries in mine. I bought them at a marine dealer. (Actually, they were out when I went there, but they were able to have two delivered to their store within about an hour.) That was 3-4 years ago. I didn't use my camper last summer, only went out to the storage facility to check the batteries once and they were good. I went back again last week and tested them. Still right up there, however I did remove them and put a charge on them since I won't be hooking up again this year. (Sold my truck and will sell the camper soon.)
I was really impressed with how well they've lasted, through nearly two years of non-use, temps ranging from -30F to 100F. This was my first set of 6V, always figuring that two 12Vs gave me additional options and were cheaper. They do and are, but they didn't last anything like these 6Vs.
We boondock 95%-99% of the time, from two nights to five nights at a time usually. In Wyoming's mountains, the furnace is usually running much of the night. If you'll be on the grid most of the time, a single 12V will last a night. If you won't be using the furnace or lights much, it'll go much longer. Go from there, but if you won't have hook-ups where you camp, a pair of 6V batteries would be a good choice, imho. They are heavy! Whew!!!!