I have had the same problem, was stranded in the desolate part of Eastern
Pennsylvania and was at the mercy of a local shop that put in a new starter,
broke the old one taking it out so I was denied the core exchange.  The bus
ran fine for about 2 moths, than it happened again near Neptune, NJ.  I had
it towed, the shop there disconnected one to the leads from the starter and
it worked for a week, I had it replaced again.  This one lasted about 9
months, then, (in PA again), it happened at a fuel stop.  The bus was fully
laden, so I had to unpack it to get to the starter (this always happened on
trips it seems), tried the hit-the-solenoid method.  It worked.  This time,
I went to the a local shop here in South Jersey and they replaced it with a
Bosch starter, that had a warranty.  So far okay.  I noticed with each
failure there was a subtle warning sign.  It seemed there was a bit of a
conduction delay when I turned the key before the started engaged.  Very
brief.  Each time this happened, the starter would fail within a week.

One should note, the first time this happened was in October, going from
Cleveland to South Jersey, I broke down around Bloomsburg PA, although the
engine was hot, the weather was not.  The next two time were in November and
December.  The last time was in July, same trip as the first, within 20
miles of the first in Nescopeck, PA.  Of course each time this happens it is
on a Sunday and I am with my wife and 5 kids!!!