This is a video I stumbled upon some time back and I made this exact installation on our trailer. The one thing I did different was to mount the camera to the top edge at the back of our trailer and tilt it where I can see the spare tire on my bumper. I also have the wireless router piece permanently mounted inside and wired into my 12v system. It has a very wide angle so I have a great view of the road behind me. It really helps when backing into a camp site also.
Do-it-yourself CHEAP rear view camera solution