Always.
Theoretically there is a very slight chance that while filling up your and your RV was sitting next to a gas pump and the refrigerator vent panel was very near that gas pump hose as it was pumping gas and the wind was exactly right to lift the heavier than air gas fumes up and blow those into the refrigerator vent and the refrigerator was currently cooling and the fumes reached the burner, then yes, a fire is possible.
Likewise if someone sprayed gasoline into the refrigerator vent while it was on ………..
Diesel fumes are not explosive like gasoline fumes. Diesel is more like charcoal lighter fluid as far as flammability.