I'd say you are in spec. with those readings.
Try it again at idle with the rear defog on, high beams on, and blower fan on high. See if it can keep its head above water under load.
'3 hours of running time in a couple of months' might say it all, though. There is substantial 'static' drain from sitting still. That may not be enough run time to keep the charge up.
1986 300E 5-Speed 240k mi.