Sounds like a bad diode. I had this on my BMW. The alternator would put out voltage/current, but the diode (blocking device) was bad, so the battery would discharge back into the circuit.

The oddity here is that youve tried additional alternators with no luck (if Im reading it right). This would signal to me a discharge of some other sort... Not an internal short in the battery (since youve tried multiple), Id guess some sort of soft short in the car wiring that is not carrying enough current to set a fuse.

Id try both with the alternator connected and disconnected, to put an ammeter inline with the negative cable and see what the load is with the car off and sitting.

The alternators used in these cars are weak by today's standards, so may get flagged in a test though they are OK. It could be something silly liek a trunk lightbulb staying on...

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)