On my second night of tests with the same conditions as the first I ran the furnace from 10 pm to 7 am. By 7 am with both the furnace and the TV on I was down to 11.5 volts. But, I was happy to see the power inverter was not signaling that the voltage was to low. With everything shut off the voltage was 12.2. This seems to be how the system always operated so I have no idea why it acted up last fall. And, I have no idea why the voltage seems so high when the battery's are at rest. But, it all works so well I guess I'll just run it. I will flip the converter / charger back on today. Charge things up, and let it go back to bed for the winter.