I've got one of these, which links to a smartphone and works well for me.
It's magnetic so I glued a metal plate to the back of the caravan - there's a small ledge just above the number plate which is horizontal-ish. At first I also had a safety lanyard attaching it to the two rear grab handles, in case the magnetism didn't hold, but I've got more confidence in it now and don't bother.
It's powered by a built in rechargeable battery so no need for wiring, I take it indoors to charge. I've never tested the battery life to exhaustion but it's still going after seven hours. I leave it on, as a rear view camera, not just for reversing. I know the legality of a permanently-on screen is questionable, but since it's showing me the exact same view I'd have from the rear view mirror if the caravan wasn't in the way, I'm happy.
I use an old smartphone for the screen, which is used for this and nothing else, and sticks to a magnetic mount which clips onto the air vent.
I use it for other things away from the caravan - when the tatty car is loaded to the roof for a tip run, I put the camera on top of the boot hatch as a rear view camera. It also works as an event camera - I do bits and pieces of tech stuff for an event management company, and I put the camera in the main event room so I can see what's going on from the crew room.