When I was installing we did a lot of them that were a "bullet-style" camera that you could drill a hole and mount in the license plate. We did one on a factory hitch and set it inside the hole that sits right above the receiver. You could see the ball perfectly, it also had a clear view to all of the surroundings.

eBay Motors: Backup "Wide Angle" Rear View CCD Camera w/Night Vision (item 270331041627 end time May-15-09 15:31:22 PDT)
one similar to this one.