If you understand how the automatic changeover valve works:
You could follow the advice at the following link to test your changeover valve. However, the link suggests using your stove for the test, instead of the furnace because your thermostat may turn off your heater (due to reaching the set temperature) before you had time to complete it all.
As Lou said, you may have enough seepage to run the stove but not the furnace. If you could make the furnace stay on long enough for the test, then that should do it.
The RV Doctor: Testing a Propane Automatic Changeover Valve