The first thing I would do is get one of these 7-way TEST ADAPTERS from AMAZON-ETRAILER-probably any number of auto parts stores.
This will TEST all of your safety lights DC signals for proper operation. Once all of the SAFETY lights (Running lights-Brake and Turn Signal Lights) are known to be working properly then you can start testing for the BACK-UP LIGHT feature.
Most often the BACK-UP LIGHTS is wired from the center connection and from GND.
Since your truck camper was previously owned it may have prior owner mods to the wiring.. Some truck campers have a second cable installed in the bed of the truck to make it easy to plug into. You may have one these installed.
Got to start somewhere tracing it all out.
ETRAILER has schematic drawings and wire colors for the regular campers - have never looked-up for truck camper diagrams...
Roy Ken
