Yes, it is the switch to the PC-type cooling fan that is mounted near the refrigerator condenser coils. It seems to be thermostat controlled; and if turned on, that fan cycles on and off (mostly on in the summer, here in Arizona).
Check other posts on this subject on replacing the OEM fan, since if you live or visit hotter climes, you'll probably get tired of the fan noise. Luckily, quieter fans don't cost much. I am replacing ours with a $14 model that is quieter and moves more air. If you live and camp where it doesn't get above 90-100 degrees, you may not need to turn this on.
Since we got our trailer in 2005, we haven't used the fan because of the noise and battery drain. However, in the 110+ degree summers, the ice cream gets soft and the lower refrigerator compartment sometimes reaches 45-50 during the day. That's why I am finally installing the quieter fan, in preparation for this summer.