Want to add a battery to my truck camper - help please

I do not have a 3 way fridge and would not even consider it.

Jim

I have a newer camper with a auto/manual fridge setting. In auto mode when engine is running the fridge is on 12v, turn the engine off and it switches to propane, plug the camper in to shore power or gen and it switches to 110v. Since operating on propane is the most efficient mode, when really hot I manually override the controls and stay on propane.