Why is an exhaust fan needed and how does it work? Well, the heat source for the refrigerator--either the propane flame or the electrical heating element--is used to "elevate" the ammonia/water mixture up to the top of the refrigerator's cooling unit. Once the mixture is there gravity and a chemical reaction begins and cooling is accomplished. Air flow is needed to dissipate the heat collected on the refrigerator's evaporator fins and from the refrigerator's cooling unit. A 12 volt exhaust fan helps to accomplish this by circulating the cooler fresh air received from the side vent and across these fins.
Exhaust fans can be purchased at any RV parts store or on Amazon.com like this 12 Volt Exhaust Fan. You can buy either battery operated fans or ones that come with 12 volt power leads that tie into your RV's 12 volt system. I went the latter route. Connecting the fan to the power source was the easy part, the more difficult problem was finding a place to attach the mounting bracket. There was no place. After looking at all the options, I decided to screw a small block of wood to the floor. You can see in the picture below how I was able to attach the bracket to it. Pretty simple! Oh, a quick note. When you install the fan ensure it's facing the right direction and pushes air up when you install it on the motor. I've come across some guys on the Open Roads Forum who installed their fans backwards and wondered why no cooling was taking place.
I performed this simple installation last spring and was able to use my refrigerator during the hot summer months with no cooling issues whatsoever. So if your refrigerator isn't cooling so well in the summer you might want to give this modification a try. It's cheap and easy and well worth the time and effort.