What type of battery you need to buy will depend on how and where you plan to use your RV, along with what will fit in its battery compartment. If you plan to just go from one full hookup RV campground to the next only needing battery power to run things for a few hours while you drive from one electrical hookup to another, then you can get by with a single basic 12V deep cycle group 24 (smallest)/ group 27, 29 or group 31 (largest) that sells for about $100 give or take at Wal-Mart, etc. These will usually be marked as RV or Marine Deep Cycle batteries. If you plan to camp off grid, things move up from there, though many come from the factory with a pair of Group 27's wired in parallel, this is just a starting point.
Depending on how much stuff you run, and assuming your RV has a RV style absorption refrigerator and not a residential refrigerator with an inverter, then you can likely get 1 or two nights out of a pair of group 27's between needing to recharge depending on how much you run the furnace and conserve power. Though daily charging is probably most common.