While troubleshooting this problem further today, I ran a test by connecting my camera directly to the coach 12 volt battery in the rear. The camera was attached to the intended mounting bracket up top and, voila, it came to life. Better yet, I had four bars on the monitor in the cab, 30 feet away. So my problem is not due to interference with the metal sheet panel embedded in the back wall. It appears to be a polarity problem with the 12 volt wiring I tapped off from the upper marker light. A volt meter, in fact, reflected -11.7 volts when I put the negative probe on the sleeve and the positive probe on the center pin of the camera's power cable. Am in process of reversing the wires that feed the camera. Admittedly, I'm a bit confused that the LED marker lights seem to work wired the opposite way. While doing a bit more research it appears that some manufacturers make LEDs that are not polarity sensitive. So, I guess I must have those, too. It's dark now and I will have to wait until tomorrow to run the penultimate test. Stay tuned.
