It COULD be the thermostatically controlled fan that cools the heat exchanger portion of the refrigerator (Is that called the "evaporator"?) Anyway, regardless of what it's called, the fan only comes on when that part gets hot enough - something over 100 degrees I believe. But, for the fan to work at all, the refrigerator has to be on and its controlling switch also has to be on.
There's also an inline fuse in that circuit that has to be good. You find all this stuff behind the vent panel on the outside of the trailer behind the refrigerator.
- Jack