Today's RV fridges are gas or 110 volts. I don't believe you can get a 12 volt fridge any more.
The fridge is more efficient on gas. It will cool down faster with a flame than with the element. Once at temperature, either system should work fine.
Gas fumes are much heavier than air. At a pump they would collect at ground level and not jump up and into the motor home. Of course there could have been bigger problems like a leak in the tank intake or some other malfunction. Since a trailer does not have an on board gas tank, the chances of a fire or explosion due to the fridge running remains infinitesimally small.