Fan inside or fan outside? These are two entirely different issues. When your fridge is working sufficiently, cold air inside the fridge has a tendency to drop. Things at the bottom get colder than those at the top. If it is too cold inside then you get frozen lettuce. Circulating the air inside the fridge is a good idea as previously posted.
If, on the other hand, your fridge is not getting cold enough then adding a fan on the outside can help it function better. Mine vents through the roof and works just fine even in the direct sunlight on hot days. Others have fridges that vent out the side of the trailer making it more difficult to draw air through the vent. Most of the smaller Airstreams today with these fridges come with built in fans.