Cari & i use one called "HindSight" backup camera. Both the monitor (about the size of a Smartphone. I place the monitor on the rear view mirror with its built in clip) and the night-vision camera are wireless. Each will go about 70 minutes before needing a recharge. I bought a simple ($20) cable so i can charge either one, but it comes with a 12 volt charger with a Y adapter for charging them both at the same time.
The camera has a magnetic mount. I will typically place the camera on the van's rear cargo door (about 11 feet from the cab) and point it downward so i can see the trailer hitch-up in progress. Then, when we are ready to head down the road i take the camera into the trailer (15ft from hitch to bumper) and have a small metal plate attached with velcro to the rear window frame to point outside as a rear view mirror going down the highway. It has worked great so far. And because it is inside, it does not fog up like our last unit which was a license