I used a Peak wireless camera system for a few years until it finally died a couple of years ago so I replaced it with a much better 
Swift Hitch SH02.  The SH is much better built and the feature I like the most is that both the monitor and camera/transmitter are self powered and require no interfacing to the vehicle itself.  I found the Peak pretty useless for watching traffic behind me or for backing into campsites as the view was so fisheye and the ability to display a decent picture with both bright and dark elements in the picture quite poor.  These days I use the 
Swift Hitch (5 pics) just for hitching up and for that it's invaluable. :B