First add the isolator inside the engine compartment so it is in line with the power feed to the camper. This will protect the truck battery or batteries which is your primary concern.
Disconnecting the camper battery with the switch will keep them from rapidly draining from anything inside the camper. They will loose roughly 10% of their charge each month when sitting unused inside the camper. You can periodically charge them back to 100% or add a trickle charger or add a solar panel to the roof. Solar is the perfect trickle charger.
Pulling the battery and taking it to an automotive supply store for testing can tell you whether the battery needs to be replaced or not. With a flooded type low water level will also keep it from charging or holding a full charge properly.
