The answer is NO!! The alternator in an up to 89 300E is a POS. It has a hard time keeping up without seat heaters.
The problem is both under total capacity and general poor performance. The alternator is probably a 70amp unit. Recent E320s have a 120amp unit. But this isn't the real problem as the 1990 on 80amp 300E alternator does just fine.
If you were watching current flow you would see why you are dropping below battery voltage. Its sort of like a bank account. If the money going in doesn't match or exceed the money going out then the pile (battery voltage) gets smaller. You either need to change your spending habits or get a better paying job (bg).
THe problem with the standard 70amp alternator is that it only produced about 25-30amps at idle speed. At about 2000rpms it will produce the 70a and should handle the load. The 90- 300E 80a alternator produces about 60amps at idle and will power all those debts.
It turns out that the later alternator is no more money than the POS. It installs similarly with the exception that one must change the $50 alternator harness. If you can't find the number in the archives (for the harness), write me on Monday, we stock the harness and never use the POS.
Bosch Master, ASE Master, L1
33 years MB technician