The Dometic fan motor draws 2.6 amps at 120v, or a 312 watt load. A rheostat (or resistor) in series with the fan will drop 1/4 the wattage of the load at the worst voltage (60V at the motor & 60 volts at the rheostat). That means you need a 78 watt resistor or rheostat which is fairly large, and could be radiating a fair amount of heat. If you're using non variable resistors to drop lower voltages, you could get away with less wattage, but it is still going to be large.
While I would be concerned with icing as Dave noted since even a 9600BTU AC is often overkill for small fiberglass trailers, if you want to slow down the fan motor, a motor speed control would probably be a better idea, although you need to shop carefully - many of them specifically state they are not for capacitor start or run motors.
Another possibility would be an autotransformer (a Variac
, not the RV Autoformer) that can be used to vary AC voltages, however that may cause a starting problem. Of course you would not need the case that is part of the Variac I linked to...