Never had an alternator problem with any of my MBs...knock wood, so not too much experience digging around inside them. However,

Alternators usually employ some internal method to energize the field so the unit will produce AC power. Your main diode assembly converts the production AC into DC to charge the battery. There may be another diode assembly inside the case that is used to energize the field coils. On older GMs this was called a diode triplet. I have had one of the diodes in the triplet go bad and that caused a weak field and only 12.5 volts of output, just barely enough to keep the system running, but not enough to prevent the battery from running down over time. It was a condition similar to yours, so maybe a similar cause?

Just a thought...FWIW and good luck.

230/8