Dead batteries in 04 Duramax 3500

Would not the plug in at the bumper do the same thing? Unplugged no draw, plugged in draw? When the truck is sitting at camp or in the shed the camper itself unplugged and the camera not plugged in-no wiring connection to truck batteries. I'm just trying to find out if, since this happened a second time, after the install and once before, if leaving that receiver and camera drew enough to run down the batteries or if it could be something else.