Most of the wireless cameras come with monitors and no output. The few I have tried that did not were wired but you can use a wireless link between them.
Look for a wired one that you like and use something like this. I will warn you that the power is weak on the one I tried as they are designed for a single vehicle but you could run the camera antenna to the front of the trailer to resolve that.
I tried this one and I would NOT reccomend it for a trailer setup as it cut out.
(hoping to try this one)
If you find something decent let me know as I am trying to use my dash monitor as well but I am too lazy to wire it 50ft.