I took the battery to Autozone for a charge and test, and to my surprise it was bad! Got a free replacement. (it was holding a charge, but, just barely 12V and not a completely FULL 12.6+ charge, thanks for suggestion sasnydley)

I then figured out why my dash battery light wasn't coming on. That light gets its ground THROUGH the internal wiring of the alternator. My alt was so fried that it was not grounding the circuit when the key was switched on - so no light!

Installed new alt and battery. Started up fine, dash light works. While running, with lights and some accessories turned on, I did a voltage drop test from + battery terminal to back of the alternator terminal. 0.2V, within spec.

Then did the same test from Neg battery terminal to the alternator case. Under heavy load with lights, seat heaters, etc., getting a 0.45V drop on that...kinda high, right?.

Then as a quick test I ran a new 8gauge grounding wire from one of the alt mounting bolts directly to the neg battery terminal. This time the engine seemed to start with a little more gusto, dash lights seemed a little brighter. The voltage drop with the same load just barely hit 0.3 V. So a good improvement.

Conclusion...weak grounds creating slightly high resistance, slowly frying my alternator and battery over time, right?

Problem is I really can't see how to reach a few of the grounds to clean them up or replace the wire. Neg cable to frame and neg cable to motor mount seem particularly inaccessible.

I think for now I'll just permanently install my extra ground wire from alt bolt to battery neg...no harm in doing that is there?