Proper Temperature
As a general rule, it is recommended foods enter their proper temperature zones within 2 hours which for cold foods is 40 degrees Fahrenheit and below. The freezer should be at 0 degrees.
Propane vs Electric
Traditional wisdom has it that the fridge cools better on propane, but there is absolutely no science behind that. Maybe it was true in the past, but modern RV fridges have good-sized electric heaters that are equal of the propane heater in performance.
Pre-Cooling Refrigerator
Keeping the refrigerator door closed allows it to cool faster, so using a cooler until the refrigerator is cool enough to use will help.
To maintain the temperature, try to pre-cool items prior to placing them in the refrigerator.
Putting frozen ice bottles in the freezer and refrigerator at the beginning of the trip will help cool the refrigerator faster.
Traveling with Propane On
Refrigerators will only drop a few degrees during travel if you leave the refrigerator door closed. Put all items you will need during your travel in a cooler.