I work for a trailer parts and accessories store, and we carried a camera called the Hindsight Backup Camera from Tow Ready. It comes with a single day/night camera with a microphone built into it(eliminating shouting matches and saving marriages) and a multi-channel monitor. It is a wireless system, so the only thing you need to do is provide a 12vdc powersouce for the camera. Additional cameras can be added on(up to 4 including the main one). We sold the kit for $250 I believe.
These can be mounted anywhere that you can provide a power source and a ferrous metal surface. Chrome plated steel escuchion plates make a nice metal surface to mount on. They can be found at any hardware store.
The part numbers are:
67300 - HindSight Single Back-up Camera System w/Magnetic Base
67302 - Hindsight Single Back-up Camera, Channel 2
67303 - Hindsight Single Back-up Camera, Channel 3
67304 - Hindsight Single Back-up Camera, Channel 4
Hopefully this gives you some insight as to which one to buy.