Few more details. My van has rear toilet room and I made a long brackey which fits along the top of the window and is held in place when the window is fully closed and locked. The cable from the camera is thin enough to pinch through the winndow seals, so no holes required in the bodyworh, and nothing stuck on to the caravan. Easy to remove camera if parked in a place where it might be vunerable. This could wrrk for twin lens camera just as well.
Suppliers assured me transmitter is waterproof and would be ok in toilet room which may get a little steamy when showering but is generally quite dry. The camera height is about 2m above road level, which enables me to reach it and clean off the lens easily after rain etc.
I preferred to take power from leisure battery to avoid another car to caravan cable with connections out in the weather. It should be possible to pick up the supply from the car which serves the fridge such that you get an 'ignition only,' supply, but in the absence of a decent wiring diagram it was easier to run directly from the battery. The switch is fitted near the mover master switch so it is easy to remember to switch off the camera when pitching or connecting up.
We have now towed around 3000 miles in the year since fitting the camera and have had no problems with it at all. The monitor fits over the driving mirror, so is shaded from most reflections and is in the place where you look instinctively before overtaking or joining another road etc. the monitor can be left in place without the caravan connected when the rear view is similar to the 'dipped' position of a dipping interior mirror, probably not suited to long term use.
My only criticism is that the connector from monitor to cable is small and less robust than the other 4 pin connectors, but just needs to be aligned exactly before pushing parts together.
I'm also lookin at fitting a camera to the car, but have yet to work out a 'no holes' method.