You will not find a modern RV fridge that runs solely on 12volts . The RV fridge will always seek 120v AC as the FORST and FOREMOST source of power, to get the thing cold. After you plug it in to 120v electrical source, expect at least 18-24 hrs (playing it safe) until the fridge has reached the optimal "cold" and stable operating temp.
Propane is the "secondary" means for optimal cooling. Starting the fridge solely on propane may take a bit longer to initially cool, when compared to 120v AC.
If by chance you have an odd ball fridge that does run on 12v DC, that is only to help keep the thing cool AFTER it was started on either AC or propane first! 12v alone will never get an RV fridge cold initially.