Depends on what kind of camping you do. If you seldom dry camp, or just for a day go with a one/two 12v DEEP cycle battery of your choice. If you like to boondock and have the space go with a pair of 6v wired in series.
Marine / Recreational Vehicle – MR | Trojan Battery Company
will list the size and they are standard across all manufactures. For the 12v go with SCS150 and the 6v go with the T-105's. They ain't cheap. Others have had good luck with Costco and the employee hating big retailer from Arkansas batteries.
This has great information on all your 12 volt needs. The 12volt Side of Life (Part 1)
Find the local dealer for Trojan's, call them and get price. Show up with cash and play Let's Make A Deal.