I am going to disagree with the other responses.
An average electric fan draws about 15 amps. There are some 30A killer fans you can buy as well as some 10 amp ones. For example:
Fans - take your pick You can buy toggle switches rated at 30 amps which should eliminate the need for a relay. (The Bosch relay everyone is fond of is rated at 30 amps)
Heavy duty switch I assume you must have already converted to 12 volt if you are considering the fan. I don't know if there are 6 volt radiator fans available.
If your generator puts out 40 amps (typical) you should be ok as long as the battery is good and you don't operate a lot of other things and you probably won't need the fan at night when the lights are on.
So I say go ahead, but keep an eye on the ammeter. We all do that anyway, right? if the ammeter is constantly on the discharge side, turn the fan off.
