Bob,
There are typically two solenoids back there, one parallels the two battery bank for emergency start which you could try and see if it makes a difference. There is a second one that goes to the salesman switch that shuts down many systems in the coach. You could listen at the solenoid and have someone run those switches to see what does what. When you say you jumpered the solenoid in the electrical bay I assume you jumpered the large leads which would have been correct. If the transmission pad lights and the dash gauges come on with the key then that solenoid in the electrical bay under the drivers seat is working.
There may be a small relay that shortens the wiring path to the winding that triggers the starter solenoid rather then using the key directly. If the solenoid at the starter clicks then the wiring path from the switch, whether it uses a relay in the back or not, is working OK. It could be the starter solenoid on the starter, the starter itself, or the heavy cable + or ground connection that power the starter. I would be looking at all the heavy cable connections between the battery and the starter to see if they are clean and corrosion free. There should be + 12.8 V or more on the large lead on the starter solenoid. If you can safely check the voltage on that terminal to ground when the key is turned to the crank position, that will help sort this out.
Later Ed