Unless you want to spend alot of money, and be unable to remove the pro fitted one when you sell your box, try this: http://cgi.ebay.co.uk/GET-Baby-Wire...-/120627412474?pt=UK_CCTV&hash=item1c15f43dfa
I ahve one in my box and its brill. You do need 2 cameras if you intend to travel more than 2 21/2 hours though as thats about the battery life on the camera.The monitor plugs into your cigarette lighter so that will run all day. The quality of picture is very good, and as its colour you can see very clearly too.
I have the trailer vision (early model) and it's great, it is basically a baby monitor and wireless camera similar to the one Spottyappy has found on ebay.
My battery had stopped holding it's charge and I have bought a converter and a long extension wire which plugs into the cigarette lighter along with the camera (cost approx £15 for both inc postage) - it gives a better picture and means that I don't have to remember to charge the battery up before going anywhere.