One thing to remember is batteries loose capacity when they get cold. Only really matters if you plan on winter camping.
If going with AGMs, I would find room inside the camper for them so I didn't have to load them up each time or worry about them getting cold. If you don't have available cabinet space, you could easily conceal them in a step or under the cabinet overhang.
As said above, since AGM can be discharged deeper, there isn't much of an advantage with 6 volt batteries. Big advantage and reason I would go with 12V is if one of the batteries dies, all you need to do is disconnect from the circuit and the other battery will still be useable.