Your post doesn't mention the year and model of your coach, that might help others in troubleshooting your problem. Since you say you have removed and reconnected all of the cables, that would seem to eliminate the possible loose connection. Is there maybe an internal fuse in the system? It would be highly unusual for all three cameras to go out at exactly the same time. It's probably somewhere in the monitor itself. I presume that you have already tried adjusting the contrast, brightness controls and the day/night switch?
You might also try www.rvcams.com
for help. I think the owner's name is Tim. Several years ago I had a minor problem with my system in a '98 Brave. I finally called them and they were very helpful in resoloving my problems. They had the parts I needed at a reasonable price.