I've got an issue I'm trying to figure out on my '79 300D.

Yesterday, she was driving totally normal. Drove through some rain, puddles; parked on a slight downhill (mentioning these in case they matter).

This morning, went to start and the engine hardly turned over. Turning the ignition, I can keep the engine turning but at a very slow rpm (like when battery is low).

Thinking the battery might be the issue, I tried to jump it but that didn't make a difference. Battery sits at ~12.25v - low but not terrible (jump it was at about 12.7). Cleaned battery connects and the ground next to the battery. Also whacked the starter with a hammer a bit. Cables from battery to solenoid seem to test well (voltage drops ~.1v)

The one thing I've found, is that when cranking, the battery reads ~3-4v which seems really low. I watched one video where the guy said it shouldn't fall below 10v. Does this conclude wiring? Or could it be battery?

Any tests I can run? It was hard to see in there real well. Is there a trigger wire in there I can test? This is odd because this issue came with no warning.

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'79 300D