I've been looking at these for the past year, and not too happy with my choices, or more precisely with the cost/features spectrum.
Dedicated backup cameras:
--Wired cameras - need to run wires, cheapest. Less interference.
--Wireless cameras - need a receiver near the monitor, don't run wires. (Could be moved to trailer if power could be worked out, then moved back to vehicle)
Multiuse cameras
I.P. cameras - have a webserver showing what the camera sees. Often used for plant security, etc. Need a laptop/netbook/etc (maybe a router too?) to display.
-- Wired, use an ethernet cable
-- Wireless, use 802.11 frequency.
All cameras need a power supply so putting a wireless one on the trailer might be a problem, unless you have a battery on the trailer (a boat or camper might).
Monitors: Do you want too small/low res., or too expensive :-( ? A cheap car DVD player monitor might work if the inputs were worked out. Dedicated backup camera monitors are usually too small for my taste (3-4 inches), I think you want a 7 inch (diagonal) screen at the minimum. And then where are you going to put it? Some better GPS units have a camera-in port, the wireless receiver or wired camera can plug into that.
I'm thinking of a wireless IP camera that I can mount anywhere - I can use my iphone to see the picture, or a laptop. Then I can use the iphone or laptop as a navigation device too. But unless I get a built-in car computer or something I'm expecting to be partly unhappy with whatever I choose. So I've not done anything.
For simply backing up to a trailer, something like this (
http://hitchingguides.com/ ) will work - or you can make your own from spare magnet-mounted antennas and tennis balls.