I have had a wireless "Peak" rear view camera that is mounted by a spring clamp to my rear widow valance inside and I can monitor the traffic behind me and if I was towing something it would work well for that also. It has a 7" screen and works great. I ordered a extra 15 ft. cable so I could mount the transmitter in the front closet and have never had a problem with getting a image of the rear view. I've had this unit for about 4 years and still works perfect and have less than $150.00 invested.
I do not think it helps much for backing but sure is a good monitor for what is happening behind you.
Kountry Traveler