I have installed one on my boat trailer, one on my previous class C, one on my truck near the ball, and now have 3 on my new trailer. If you have nothing (no monitor or double-din stereo with video input) I would recommend http://www.visorview.com/
I started with their wired system. You have the option of a simple cheap monitor or putting one into the visor of your vehicle. The reason I went with them is they guarantee the cameras can be submerged on boat trailers. I went with a two camera system (truck/boat trailer). I still have that setup - transferred the camera to my new boat trailer - and have had no issues.
With my new trailer - I added one camera to the back, and one camera on each side of the trailer. I went with the cheap amazon cameras - http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
They are cheap, cheap, cheap - i ordered 5 just to be safe - for when one fails.
I also upgraded my truck stereo so I didn't need the monitor anymore. It also allows me to switch from truck to other camera - I now have a multi-switch under the stereo in a cubby hole. I don't really like it much - came in army green but was supposed to be black - http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
Fits the spot - now I just select other input and can push a button on the multi-switch to see any of the three cameras on the trailer. Normally, I just leave the backup camera on and drive down the road - had to lock out the parking break wire so I could watch as I drive instead of only when I backup.
To answer your question - I would stay away from wireless setups unless you are going to use it on a farm by yourself all the time away from anything else. Otherwise, you will see baby cameras as you drive down the road or just get interference as others have said.
If you want to install something like this yourself, choose a setup that has the power and signal wires in one waterproof cable like the visorview stuff. Otherwise, you will have to run separate power and signal (RCA) cables. That will make it an easier install. Most good cameras (not the cheap ones above) will have these good cables (9-pin din if I remember the name correctly). The other side of the cable will have 9-pin to power/RCA cable (power to your vehicle fuse panel and RCA to the monitor). Sounds more complicated than it is.