Jack, check out my reply on LOA, I included a pic of where the wire bundle is located (tucked in to the frame just behind the bumper).
Here's an update, sort of, on the camera dilemma. I turned off all apps and electronics I could in order to see if there was an interference issue I could eliminate; no luck, no change. However, I did manage to end up with a blank Navigation screen. I called OnStar and they rebooted it; all is good on that front. But while we waited the 5 minutes for the reboot, I explained my camera issue to the OnStar tech support person. She said it is probably an incompatibility between my camera and the truck's IOT Radio System, which is GM's high-end system. She suggested that I could probably solve the problem by downgrading to the IO6 system, though it would be expensive to do so. I suggested OnStar do a software fix from their end instead. She laughed at me. "IOT", I assume from my Google search, refers to Internet of Things which I have seen some refer to as "Interference" of Things.
As to accessing the wiring in the trailer, I'll be checking with Leon soon, after he's had his coffee.