What you want is a surviellance system to display multiple cameras at the same time. Costco had a small b&w system for a reasonable price but you'll need a converter to run it.
I have a number of 5" diagonal flat panel color displays that you can hook a simple camera to for a backup viewer. The displays are $65 and a simple camera can be had on eBay for under $75.
I mounted a display in the housing for a PSG-7 SHTU and it worked great. I never installed it because I mounted a Panasonic Toughbook computer on the upper radio shelf and I feed it a signal from a small USB camera that attaches to the back of the HMMWV. The computer does all my GPS mapping along with MVCC rosters and HMMWV manuals besides the camera. I have the Toughbook CF-M34 on it's docking station with a power converter installed in the base that allows me to power it all of of the vehicles 24 volts.
FTM2 USS Long Beach CG(N)-9
US Navy, 1969-1975
1986 M996A1 HMMWV ambulance w/ Kasel M101A3
1977 M880 Dodge Crew Cab
1966 M274A2 BMY Mule
1965 M151A1 Mutt w/ Stevens M416B1