Camper and RV fridges that use ammonia for the cooling takes around 24 hours to fully get cold.
So I would recommend plugging your RV in for around 24 hours prior to leaving on your road trip and then you can switch the camper or RV fridge over to the propane to keep the fridge cooled down.
An empty RV fridge usually takes a bit longer to cool down fully so adding some food to the RV fridge can help it cook down much faster.
But you should notice the RV or camper fridge get cooled down to where it's supposed to be in 24 hours and if not then the heating element in the RV fridge that heats the ammonia could be bad or the flame from the propane may not be lighting properly.