What type/brand/model is it? A regular RV type 2 way fridge? Are you checking with a thermometer?
1. They can be very temperamental if they are not level/plumb. Often they are not installed plumb inside of the RV, so even if the RV is level, the fridge will not be. Check by putting a torpedo level on the freezer floor.
2. Try adjusting the thermistor. That is the little plastic thing attached to the metal fin inside of the fridge in the back. Like a mountain climber, move it up for colder temperatures.
3. Does it cool any better when running off of 120v rather off of propane? If yes, there still might be something going on on the burner side of things.
4. They really need to be vented well, so make sure the airways are clear to the roof or upper exhaust vent area.