Here is what I did with mine. It is a computer fan and it runs anytime the fridge is turned on. It has been in there since 2004 and still is working great.

This is what I have mounted outside in the fridge compartment. It is thermostatically controlled and operates when the temp gets above a certain point in the compartment.
It blows air up towards the roof from the burner assembly at the bottom.

Together these two fans have kept our refrigerator working great for almost 10 years in the hottest weather. We also try to park so the fridge side of the trailer is in the shade when possible.
Barney