Matt:
  Have you tried cleaning [what I think is] the relay connection at the
starter?  Crawl under the bus and find the starter.  At the top of the
starter (above the big post for the direct line to the battery) is a
smaller red wire with a push-on spade connector.  Clean that connection
with the sandpaper / solvent / whatever of your choice, slide the
connector back on the post, and then try starting the bus.

Best,
d.
1990 GL -- "Mudskipper"