This has nothing to do with your switch question actually. Across the whole spectrum of RV types the 12V option for the fridge is rare as it is very inefficient in the absorption type refrigerators. For your purpose, if you are running the fridge on 12V battery power and they have provided a fan, you should probably turn it on. The bigger question is why would you run it on 12V? When shore power is available you would likely use that mode as it would save propane and your battery. When not on shore power or an external generator propane is more efficient than 12V. Only if you were out of propane or were trying to conserve propane for some reason would you use 12V. Carry an extra propane bottle and don't use 12V then the switch is irrelevant.