The only stupid question is that which is unasked...
Do you only have power when your camper is plugged into the truck with the truck key 'on'? Not sure about other makes/models, but my Ford powers the 7 prong trailer plugs (+) only when the key is in the 'run' position.
Did you see any wires hanging down when you put the camper on the truck?
With mine, a 78 model year, the pigtails for the battery hook ups ran down and out from under the kitchen counter cabinets into the front left bed of the truck. There was an access door where you slid the battery out of the camper and into the bed of the pickup and hooked it up to the battery there... the 'battery mount' was the bed of the pickup.
I'd do as much poking around with a flashlight as you can, following the wiring from where the pigtail enters the camper. The black(+) and heavier gauge white wire(-) are the ones that will lead you to where the battery should be. Keep your eyes out for two wires that have a battery terminal connectors on them and aren't hooked up to anything inside of the cabinets and compartments.
I'd be very surprised if someone wired the truck camper only to be powered when it's plugged into the truck's DC batteries. Automotive batteries are not deep cycle and aren't designed to be used in a large draining and recharging cycles like campers, boats, rvs, and golf carts.
If you want more info on how everything works(or is supposed to work
), just ask.