This is one of those must have items if you have a large RV, whether it is a big Class A or a 5th wheel, they are one of the greatest assets you can have on the road. In august of 2015, I purchased this unit from Amazon. It is a wireless unit which worked very well for the greater majority of the time, periodically it would go blank for a few seconds, but otherwise performed very nicely.
This link from Amazon shows the unit I purchased, which has two cameras, I mounted one of the two cameras just above the rear window of my 5th wheel and have it covered by the overhanging area above the window. This helped a lot in keeping the camera clear of rain and dust, and also provided a nice clear view of the road behind the rig when I was making lane changes. It does help a bit when backing up but not near as much as having someone else on a cell phone watching out for you and giving you instructions.
The biggest advantage of the high mounted rear camera is making sure that when you do a lane change you have cleared the vehicle you may have been passing.
Rear View Backup Camera System, 7″ Digital Wireless Split LCD Monitor with Two Wireless Waterproof Ir Color Cameras for Excavator, Cement Truck, Farm Tractor, Trailer, 5th Wheel, Rv Camper, Heavy Truck