Gary has a good idea,it you can connect the camera to the monitor inside the coach,you may need an adapter of some kind to make the connection(s) That will tell you if the camera is working, it it is then its a cabling problem.Should it not work it could be an intermittent monitor or camera issue.
If you have some other camera to connect,good way to eliminate the monitor.
The catch is is your BU camera an analog unit or digital? the monitor will need to see the proper format to display an image. Always that if,maybe thing!!