Check the connector crimp on the alternator end of the larger gauge wire
from the alternator to the starter.  Once I had corrosion here and it gave same
symptoms, i.e. unexpected failure of the charging system.  It took a while
to diagnose since the alternator appeared to be charging when measured with
a volt meter.  The connection looked OK but wasn't conducting the necessary
current.

Note that this is not the huge Starter to battery cable but the medium heavy
gauge red wire that carries the alternator charging current to the starter
terminal and then on to the battery via the huge battery cable.  Be careful
not to short out this connector since it is always "hot".

Ed