My under $100 (on sale) back up camera from an auto parts chain works OK for the money. It picks up interference every now and then but is mostly reliable. It is great for lining up on a trailer hitch. For more extensive back up use I might step up the quality.
I mounted my screen on a shelf I installed that runs above the windshield, behind the visors, so I don't have to hide it from thieves like I do with my GPS. I also plugged into a 12 volt outlet with a switch so the monitor isn't on all the time, only when I need it. The camera is wired to the back up lights.
I want to add a camera to the back of the trailer and am considering options. One has two cameras and one monitor which shows the truck view when disconnected, the trailer view when connected. Whatever goes on the back of the trailer I have my doubts wireless will cover the distance but, many folks have had it work (with more expensive set ups than the $100 kind). If the cheap wireless I have would work, I might consider putting a plug connection on the camera / back up lights connection then, unplug it, move it to the trailer license plate and plug it in there.
DRAGONFLY - 2011, Provan Tiger CX, on a F350, 4WD, regular cab chassis