Some additional info on the alternator warning light.  IT WILL NOT ALWAYS
LIGHT WHEN THE ALTERNATOR IS NOT PUTTING OUT VOLTAGE.  If the brushes fail
in the proper mode there will be no charge going to the battery but the
warning light will not light.  I had this exact problem in January, my Westy
ended up dying in the middle of I-8 outside San Diego, ran on jumpered to
the aux battery.  Replacing the regulator/brush pack cured the problem.  And
I've seen this happen on alternators other than Bosch.  So one of the
electric gurus out there can probably explain exactly how this failure mode
works but it can and does happen.