You can go with either wet cell of gel cells as your choice. You do need to use something that has a deep duty cycle as opposed to a standard car battery. If you have an internal side compartment, you are severely limited by the size of the compartment. Measure how large you can go and put the largest battery you can fit in the slot.
If you are considering doing dry camping or boondocking, you will need to go to the two 6 volt golf cart batteries to get longer battery life. One battery won't do much good for dry camping. The two batteries, will require you finding a place to mount them.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot