I have an older VR3 wired camera I got from Pep Boys years ago. I extended the cable and mounted the camera on the back of the trailer. I mounted the monitor next to my rear view mirror. the connection is just inside the barn doors. I have it hooked up so I can turn it on and it stays on. I use it as a rear view monitor. the screen is tiny, but it beats not seeing anything at all. I can see when I pass a truck and then can safely change lanes. I think I paid $75 for it.
You can shop on Amazon for a cheap camera $20-$30 and a decent size monitor maybe between $75 and $100. comes with all the connections you need, the only thing you would need is a longer video cable and extending the power wires. very easy to do.