I recently bought this one:
Amazon.com: Swift Hitch SH01 Portable Wireless Camera System: Automotive
I see however, that the price has been increased - I paid just under $161, and it's now $199....
I also bought the triangular suction cup fixture from the same vendor that allows you to mount the camera on a window, or other smooth surface that's not magnetic...
I can 'stick' the camera on the back of our AS to check when backing into a campsite, etc...
The receiver has a small screen, but it's quite usable for backing up - I've used several times, works great!
The portability of this unit also allows for other uses that might require visual images in tight quarters...
It will operate for about 3-4 hours on a charge, so the other night I used it to watch some raccoons raid our cat food dish outside!