Grounds are good, one battery was fine 12.6v and the other was at 2.6v so I refilled the one, waiting to test it. Could it possibly be the battery isolation switch? I checked all fuses on DC panel except for the master. The master switch is something like a relay? I can’t tell but I’ve never seen one before.
Yeah I’ve got the original switch on the DC panel, how can I tell if it’s bad?This sounds like the perfect time for the main battery switch in the DC panel to randomly fail due to age. Do you have the original switch in the DC panel? It's usually a Seadog glued to the back of the DC panel with just the shaft coming through to the front. Lever slides on the shaft. Custom Ericson graphics on the panel front. Lots of main switches have failed.
Also, if batteries are combined 'Both' or '1+2' position on main switch with one 2.x volt battery and one good it may very well act dead.