Tom, did you get anywhere with this back up camera install?
With the help of Mustang Don there was no solid idea to make the camera come on by plugging the cable into the truck.
Mustang came up with the idea of using a 1 channel on/off wireless control (around $15 on Amazon). Keep one transmitter in the truck and one in the ground control controller box, to turn the camera on or off at the trailer.
Good plan, and sort of high tech, and convenient if you forget to turn it on while driving.
For me, I would end up losing a remote and without installing a little LED light to show if the camera was on or off, I wouldn't know if it was on or off.
So I kept it simple and installed a illuminated rocker switch beside the controller. I can't hook up or take the trailer off the truck without opening the little door. I will see the switch and turn the camera on or off.