David,
My coach (2000 Marquis) did about the same thing and it turned out to be a loose nut at the battery cable on the starter solenoid. The loose contact had arced so much that the nut was welded in place and could not be tighten or removed. The only way that I could repair it was to remove the starter, which included breaking the end of the solenoid because I could not unbolt the cable, and replace the starter solenoid.
It is not to hard of a job and the average mechanic can do it in about an hour or two with the right tools. I did mine beside the road in Arizona with hand tools.
Gerald