I have a rear-view camera fitted to my VW Hi-top (camper) - very useful when reversing into tight spaces and for backing up to the hitch. This is wired through to channel 1 of the 2-channel monitor. I also have a camera on the rear of my caravan - wired as far as a wireless sender in the front locker, which sends the signal the 5 feet or so to the receiver in the rear corner of the motor caravan, thence by wire as before, but to channel 2. (This system avoids the potential problem with interference with long distances and lots of metal between the wireless camera and receiver. When solo, channel 2 can pick up various security camera broadcasts as I drive past shops, etc., but once the van is on the back, my transmitter overrides any other local signal.)
As for legality, it is perfectly acceptable to use a monitor as you would a mirror - that is to say, to give you a view of the area immediately round your vehicle to assist your driving. (but you can't watch a video on it just because the road's empty!!!) It is not correct, as some people suggest, that cameras can only be used when reversing. (That's a fat lot of use if you want to see if there's a sneaky motorbike slipstreaming you on the M25!)