12 volt back feeding to pickup

RReefer madness, Any progress or report back of problem fixed?

I would open every connector for an inspection. You have a wire that has come loose and touching something it is not supposed to. You could spend hours with a test light, ohmmeter, or voltmeter and not make sense of it.

Are there any clues left out? Such as a recent repair, battery change, something repaired?

If I were to do the troubleshooting the first place would be to start with things unplugged and a 12 volt test light to check out the truck side of the path.

You can use the test light on the pigtail to see if something is on one of the pins that should not be present.

There is no magic to a simple trailer setup. Using a test light will keep you from getting fooled by a digital meter.

A quick way to see if you have a bad ground is to grab a set of jumper cables and connect the frame of the truck to the rv frame. Bad grounds can cause a myriad of issues but I have a feeling you have the rv battery back feeding though a bad connection. Inspect for worn, dog chewed wires.

Moved from Forum Technical Support

My toad back fed into my Mh with the ignition off on the MH. Was causing problems. I solved the problem by placing high amp diodes in the line to prevent the problem. Unfortunately the germanium diodes are almost non existant now with a .2 volt drop. I use silicone diodes and it is .6 volt drop but unnoticeable. If you don't want that voltage drop install a high amp continuous duty solenoid controlled by your ignition switch. Or buy a Ford with it already installed.

your pigtailis going bad and when u pluged it in the hot and runninglights touched.

Verify that the truck wiring harness is correct to trailer connectors.
Verify that the trailer is wired the same way.

It is an idiosyncrasy with GM trucks caused by not having an automatic disconnect when the engine is off.