I just got the following VOM measurements at the battery posts. FWIW, I also measured at the battery terminals and against the chassis under the seat and the readings are the same. Terminals and cables look like new, at least the visible parts.

12.24 engine off
13.77 idle, no accessories on (except dome light)
13.76 a/c on, drops slowly but steadily to about 13.25 after 15 seconds, will continue dropping if you let it (I didn't)
12.65 a/c on, head/fog lights on, drops quickly to about 12.30 after 30 seconds, will continue dropping if you let it (I didn't); raise engine RPM to 2000 and it eventually hits 12.77 and holds steady

14.02 idle after all of the above, shut off all the accessories

Any opinions?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280