Problem solved!
I took the Oliver to a highly-rated solar installer in our area and he solved our problem.
Galway girl was correct, the internal fuses (there were two) in the Xantrex inverter were blown. The service tech had another identical inverter on hand (he had replaced it with a Victron) and used those fuses, since he didn’t have new replacements in stock.
We also had him replace our AGM batteries with a Vatrer 12V 300AH Bluetooth LiFePO4 Self Heating battery (only $599) and a shut off switch, so I don’t repeat my reverse-polarity blunder when it goes in storage.
Thank you all for your help. Heading up to northern MN for some mountain biking next week.