On some Treks there are two outlets behind the refrigerator. One is for 120 volt shore power and the other is for 120 volts inverter power. The inverter power outlet is for the ice maker. Some people that do not use their ice maker will switch the outlet so that the refrigerator will run off of the inverter while driving.
This saves a very small amount of propane but some people do not like the fact that propane is in use when they drive. If you want the unit to run on propane, I think all you would have to do is move the 120 volt plug to the other outlet and push the front controls on the refrigerator for automatic operation.