If your camper's rear panels cover the tail lights on the pickup when installed, it must then have a harness that would be plugged into the truck's wiring system...IE to operate the tail lights, stop lights, turn signals, and a backup light...
This same harness would then be the primary power source for the camper's 12 volt lights, water pump, etc - IF - there's no actual compartment for a separate battery in the camper it self...
Most modern truck campers have their own battery (s) installed, with the harness for the rear lights also incorporating a POS wire that's energized from the truck's charging system through a relay to recharge the camper's battery while on the road...