Darl, after all of your posts and everything in between, even with reading them carefully, it is difficult to follow what --IS KNOWN-- and --WHAT IS NOT KNOWN--. I think only you know that for sure, so let me list a few things in order for you to go through a process to find out exactly where you are and what is not working. When you find what is not working then you must go through the same process with that segment and find out why!!

Does the key(start switch) give a signal (complete the circuit ) with the starter solenoid?

If the key does work, does turning the key activate the solenoid?

If the solenoid is activated, will it in fact actuate? (test to prove the solenoid is not burned out)

If the key works and the solenoid is in good condition, is the start signal available from the transmission?

Can anyone offer any other (start permitting signals that must be confirmed)

For some of this testing it is easier with an assistant to confirm sounds and test lamp/voltage readings from opposite ends of the bus.
If you can not get an assistant you can make up a buzzer or horn that will give an audible sound that you can hear across the bus, or a test light with very long leads.

I think that you can make tests for the trans. signal (all clear to start) in the left front compartment.
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Go back to the beginning with your thoughts.
This was all brought on by the start key being left in the -ACCESSORY- position. Other than the drained batteries and the inability to start have you found any other consequences of the, accidental key on condition?

Something seems to be burned out, open circuit, or arced closed????

This sounds simplistic, but I am not implying that it is. Each segment has its own set of complexities that must be understood and verified.

I didn't get into making anything up yet, but, well you have suffered enough!!!!!
Good luck JIM