[Richard]

I ran across a similar thread on a bus conversion forum.

I thought one of the posters gave some good advice. Batteries, ground, connections.

The similarities were that the coach had been sitting for a while. Batteries go down, grounds corrode, and connections get wonky. I know you said you checked many of those, and I am not trying to be a smart aleck. At this point it may be a worthwhile effort to REconfirm those three before going any further.