There are two different fans that assist in cooling an rv fridge,
1) inside fan which you can purchase from most dealers it is blue in color and sits inside and helps move the air inside
2) exterior fan which will PULL air over the coils in the back of the fridge to help in cooling the fridge, If you are going to add the fan to the rear of the fridge your best choice is to get a 120mm computer fan (they are all 12 volt) and make a bracket at the top to the coils in the back of the fridge and there is wiring help on the net to wire it into the needed circuit.
Both of these fans do help and if you camp in weather over 85 degrees you will need then both.
2010 Cedar Creek Silverback 35ts
2001 GMC 2500HD w/ Allison & 8.1