So that tells me the wiring is good. Keep in mind, a multimeter and resistance drops are not the best way to test these wires, but the best way I can. You could have some fraying in the cables somewhere that is not effecting the multimeter, but would a page current. I don’t mean to send you down a rabbit hole with this checking grounds.
I really don’t understand why you’re getting all these different readings on the batteries with them off. 12.4 to 12.9 is a big difference. My guess is still a battery alternator combo.
Well said Chris and Jack! Took awhile but the issue was identified.He'll get a high reading just as the charger comes off.
When you say gauge voltage vs voltmeter, what gauge are you talking about?
Some of the inconsistencies you are giving can be a cable issue, again I need to see what the cables on the terminals look like.