Could give you a certain answer if I knew what kind of fridge you have (I'm pretty sure Puma doesn't make one) but with MOST of them when it's used correctly it will automatically switch. Of course you MUST have LP in the bottles/tanks, and the valves must be open and the fridge MUST be able to run in LP mode (clean burners, good spark unit or pilot light lit) and everything in proper working order.
Wolfie, mine does. This past week while camping the 30 Amp breaker on the power source kept kicking off...fridge did what it was designed to do. I finally just went out and got my 50 Amp adapter out and hooked into it. I check mine every now and then here at home. Power on...propane on..then the power off. We just love our Puma 25RS.