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Not sure I can help but here are a couple of thoughts.
When you are not plugged into AC and the battery disconnect is off you have no 12V to the furnace control board. When you plug in to AC the control board will receive 12V from the converter as it would if you were not plugged in and turned the disconnect on.
If there is somehow a call for heat from the thermostat and the control board receives 12V from either source it will try to light, the fan will come on, the valve will open and if the furnace does not light the furnace will try to start a few times then will shut off, this shuts off the fan and if you could see the LED on the furnace control board you would likely see an error code.
I assume your propane may have been turned off when you plugged in or turned on the disconnect, if that is the case the furnace would not light and the condition above would exist. You might try turning on the propane and light the stove to make sure propane is going through the system then try turning on the disconnect, if the furnace lights then it's control board must be receiving a call for heat from the thermostat.
Since I don't know how your thermostat works I cannot advise on it's settings, but you could disconnect the thermostat and see if the furnace fan still comes on when you turn on the disconnect. If it does not, then that would indicate either a problem with the thermostat settings or the thermostat itself. If it does come on then I would suspect the control board on the furnace.
I hope this might help in determining the problem.
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