I purchased a 5 inch monitor and CMOS camera from Brash Imports about 2 years ago.  Works well.  Camera is mounted under the bumper about 30 cm to the side of tow hitch.  You can see the trailer coupling when backing and when it starts to disappear you are pretty well on the spot.

I note that Brash Imports now sells CCD cameras which are reputedly better quality than CMOS.

My system is wired which took a bit of fiddling around.  Maybe a wireless one is worth considering.

Google Brash Imports.  I went through e-bay but I think they also sell direct on line.

I recently bought a reversing camera for my Navara and the screen is imbedded in the replacement rear vision mirror that's included in the kit. The camera also has an inbuilt microphone so you can hear as well as see what's going on.

I have found mine to be invaluable in backing up to the boat trailer (or any trailer, for that matter...as well as giving a good view of what's behind the vehicle) without the misdirections I usually get from mrs. gates! Once you get a handle on the fisheye nature of the image on the screen, it takes all the guesswork out of reversing. 

If you go with one of these, make sure you order the optional wiring harness that allows you to activate the camera while you're driving along....handy for seeing what's behind you when in traffic, or for checking on the trailer/hitch while towing.

Check them out here: http://thecarkitcompany-px.rtrk.com.au/index.php/products/parking-sensors-a-reverse-cameras

Hope this helps.

regards
rusty...