I've had a twin camera now for two years.
http://www.parkingcameras.com/store/home.php?cat=19
But don't but this one, as both camras are 90 degrees, 90 is perfect for a rear view camera, but not for a reversing camera, IF you want to see both back corners of the caravan.
In hindsight i would have bought a wider angle for the reversing.
This is the monitor i bought, look alikes can be bought of ebay, whether they are the same i don't know.
http://www.parkingcameras.com/store/product.php?productid=138&cat=0&page=1&featured
I actually bought a kit with the extension leads for around £200, no doubt it could be bought cheaper, but the sony ccd is very good.
Mine is roof mounted following advice, and i do think its the ideal place to fit it, it get no dirt on the lens from road spray.
Mounting wise i used some cut offs of upvc facia, i used a flat piece with six brass set screws 2ba, which point upwards once the plate is fitted, its invisible from ground level, its secured with sikaflex.
Onto that i've fitted a plastic junction box from toolstation, and a L shaped piece of upvc to mount the camera bracket too.
The two cables, which are only around 5mm in dia are then run down the awning rail, and held in place with little bits of plastic and tiny bits of clear silicon, the cables are only seen when going from the roof to the awning rail, due to it being a Alutech, i could have drilled the rear light panel, but didn't want to risk any warranty issues.
The camera supply is taken of the battery via a 1amp fuse, through a isolating switch fitted under the fixed bed, accessible from the external locker.
Coaxes are then tie wrapped under the caravan, and terminated into a 12s plug, a 12n would be ok, its just what i had handy.
On the car i've run the two coaxes from a 12s socket, (my other one is a 13pin), through the car sill trim to the front well, from here the monitor is connected, the monitor sits in the top centre dash locker (i have a xtrail), and takes its supply from 12v socket.
I would never want to tow without it now, once you have had one you realise how many cars and bikes are unseen through the wing mirrors.
The biggest problem todate is light reflection on the monitor, so i don't recommend the monitor for that reason, i did try it on the mirror, as thats what its designed for, but i seem to have trouble focusing on it, i do need reading glasses.
On my next car i will try the mirror position again, or a position available lower down, but on the dash mounting is ideal for just giving a quick glance, so gives the least distraction from the road ahead.