There is quite a lot to know about rear view cameras. Reading the forum posts can really get lots of bad/misinformation. There is a limit as to how useful an inexpensive camera can be. I have tried the wireless cameras from box stores. In never had a positive experience. Unless they have really improved the wireless cameras in the past year or so, I would not even consider one.
My suggestion is to call a store that sells RV Cameras. They sell many different Monitors and Cameras. A Monitor screen the size of a credit card is pretty much useless. You can put a non-night vision Camera in the useless box also. I have found there is a compromise with selecting a camera with a wide angle, and one that gives you distance. You need the balance. I want to see behind me on the highway, and also see my Samurai when towing.
My system does that, and cost between $300 and $400 in 2007. Mine is similar to
5.6" Color System with Audio & IR Night Vision. I can, and have, used two cameras with it. The monitor allows you to select. My system is on when the ignition key is on.
I purchased my camera from
RV Cams - (877) 378-2267. They are great in educating you on the differences of various Monitors and Cameras. Very easy to talk to. If you do a search on RV Cams in RV.NET Forums, you will see many happy customers. I have had my system for over eight years. I would definitely buy from them again.
Wayne