The rear camera on my MS is powered through the connection on the truck, too. (12v/grnd/video/spare video) The DRV factory installed camera cabling runs from the camera connection forward (through the roof) to the pin box but, you should be able to run the cabling down the rear driver's side and under the RV to the pin box.
My camera/monitor is a "Zone Defense" system. Fantastic people to work with. Very good customer service. Had to rewire the truck recently and not only did they point me to a source to purchase their cabling but, helped me with what I needed. Customer service also assisted me along the way to be certain everything worked correctly. Zone Defense comes highly recommended.
I am so impressed with the company's customer service that I am going to add two side cameras (left and right) and a 4th. over my license plate bracket. Their website is
here. I purchased all the parts needed for my retrofit from
this reseller online (they were also great to work with).
Hope this helps you some.