Pretty much all of the higher end GPS units have video inputs from the last few years. Garmin, Magellan, and Rand McNally
Magellan 9145 and 9165, Garmin DEZL 760, and the Rand 7720 all have a back up camera feature as well as RV related features.
Source
http://www.doityourselfrv.com/rv-gps-comparison-review-best-options/ if you want to read up more on them.
As for the backup camera. Most of the GPS I have seen accept a 3.5 video jack or a RCA plug. Some require special connectors.
You should be able to stay well within that price range as long as your going to hard wire it. If your trying to find a wireless camera it could be a challenge trying to retro fit unless you go with the Magellan wireless GPS camera kit. The same folks have a write up on the different
back up RV camera options as well inlcuding the wired and wireless kits.
For that price you could almost buy a quality standalone GPS and back up camera that would be easily upgradable for more cameras. Though how many screens can you really fit in the cab?