Putting a fan inside the frig helps equalize the temps in the fridge.
Putting one in the vent stack to pull hot air out of the fridge vent area and out the top will help the fridge maintain better temps when it is really hot.
Most fridges in RV's have the ability to keep temps in the area of 20-35 degrees cooler than the outside air temps when then get over 90. The movement of air caused by the fans helps draw in cooler air in the vent slats on the side of the RV and around the back of your fridge in that compartment.
Not going to be dramatic in sub 90 weather but makes a big difference in 90+ and 100 degree weather.