I'm reading/assuming that the trucks charge line is live.
Though didn't say whether or not its connected at truck engine bay fuse block, my understanding is the Chevy charge wire isn't connected from factory.
Its the only route that 12v can be drawn from truck hence my assumption charge wire is live. On the Chevy the charge line is always hot if connected. I would meter the receptacle camper is plugged into for constant 12v. Somewhere somehow, camper is connected to the trucks charge line.
AS trailer doesn't have a charge line (odd) and pigtail to camper, charge line isn't hooked up- yet camper is draining the truck battery. Seems you are NOT using truck to recharge trailer/camper while on the road, one option is to pull fuse on trucks charge line.