If it's the red lighted switch next to the door, it operates the thermostatically controlled box fan that draws warm air away from the fridge to eliminate hot spots in the top reefer compartment. It only comes on when the compartment reaches a certain temperature, and cycles on and off as necessary. As I understand it, the fan isn't essential, but it does make your reefer run more efficiently during warm weather. While turned on, mine ran almost constantly during the summer, but now that it's cooler weather the fan isn't necessary. If you're headed to cooler climates, you probably won't need the fan at all. A common complaint is that the fan is noisy and some people have replaced theirs with a 12 volt
computer fan that has a quieter decibel rating. I've been thinking about doing that myself. I saw one at Circuit City that had blue LED lights built in. I wonder how that would look.
Regarding the use of gas for the reefer: I leave mine on auto most of the time. That way it kicks over to gas whenever there is a power loss, which came in handy a couple of times this summer at home and in a campground. It's a personal choice, about which there is much debate. I have had my Bambi since June and have been on several short trips, have gone over 2,500 miles with the reefer on gas most of the time, and am still on the first tank. When pre-cooling the reefer, it helps to load it with some ice or frozen food to get it down to temperature faster.
I recall seeing some threads that discuss both the fan and driving with the reefer on gas issues. Have a safe trip.