My previous caravan had a rear washroom window. I made a lightweight sheet steel bracket so the camera hung from the window "ledge" and was held in place by the rubber seal. The wire from the camera went through the caravan just lying on the floor, to a front bed locker where it then went through a ventilator, under the floor, forwards. When not in use the camera and wire were just gathered up into the locker.
My current van has no rear window so instead the camera is mounted on a bracket attached to the underside of the caravan floor right at the back (obviously!) and the wire is cable-tied to other wires (road lights, etc) to go forward.
Despite being low down it does not get as dirty as I thought it might. Also, this camera, (I bought a new one), is a narrow angle camera. Most reverse cameras, and my previous one, are 120 degrees: this is 60. Gives a much better idea of distances. I bought my kit from Parkingcameras.com The camera itself is CAM061Narrow and costs £60. Not cheap, but a cracking good camera.
By the way, I have mine set up as a rearview system, not just for reversing, ie on all the time - no blind spots!
Mike