biggest PRO/CON I can think of is off-gassing. If you are going to have your battery mounted inside, you need to provide a method for the gases to vent to the outside of the camper. If the battery is going to be mounted on the outside, the only concern would be to keep it away from any potential ignition sources on the outside. Otherwise it becomes a question of mounting options, and wiring supplies to connect the battery to your system.
I chose to mount our battery inside. This option helped to reduce the tongue weight
, and put ballast weight
on the opposite side of the camper from the stove/oven. Wiring for me was a minor factor because I was completely re-wiring the camper anyways. The battery box was attached directly to the floor of the closet that was destined to have a vent anyways. The previous owner had installed an AC unit that we chose not to re-install. The extra space from the over-sized double vent was almost a perfect fit for a hatch door we found on e-bay.
I have seen some battery boxes that were completely sealed, except for a tube that was plumbed to the outside of the camper to allow escape
of the battery gases.
Of course a sealed battery option would eliminate the gas problem.