I have a $100 license plate mounted camera from Advance Auto. (Sale price by the way). If the primary purpose is backing into the trailer hitch and getting a peak over the high Ford tailgates in tight spaces, this set up is a good deal. I could buy three of them for the price of the more expensive brand.
On the other hand, the wireless is subject to interference. It is a cheap system and I don't expect its life to match the more expensive models.
I just posted a question on a trailer mounted camera. There are some systems that have both a truck mounted and a trailer mounted camera feeding to one monitor. When you plug in the trailer cam it over rides the truck cam. I have seen these both wireless and wired. I will probably go this route if I can figure out what to do with the perfectly good license cam I have.
Here is the link to my post on truck/trailer cams.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27689479/gotomsg/27689556.cfm#27689556