There was an issue with the classic body style trucks and some of the wiring rubbing and shorting out. Are you using the trailer wires that Chevy provided to install the bed plug? If not, there is a bundle of wires zip tied to the frame on the drivers side, right where the cab meets the bed. It is possible there is moisture in the bundle of wiring and it is causing a short. When I put my P3 in my truck, I would initially get an alert about a short in my brakes (don't remember exact error now) and that bundle of wires was the problem. But in my case, the error would go away and brakes would function when I hooked the camper up.
With a multimeter I figured out that there was just over 2 volts feeding back over the "blue" wire to the brake controller and it was seeing a short in the system.
