if you are wanting to control the rpm to lower the drain on the battery and extend the battery life, don't use a resistor.
if you want to lower the rpm to reduce noise then go ahead and use the resistor.
when using the resistor the drain on the battery will be the same as it would be if you didn't use the resistor, so you might as well let it run without a resistor and get the most cooling you can from it for the amount of battery power you have available.
if order to lower rpm and reduce battery draw you would need some sort of electronic pulse width controller for the fan, likely hard to find there in your local city?
i am not sure, but there might be some speed controllers used on modern cars that are pulse width modulation controllers, most you will find are simple 3speed switch units that use a resistor network and mount it in the airduct to cool it and keep it from burning out.
fwiw, i would let the sucker run full speed if i couldn't locate or build a pulse width modulated controller to reduce the speed and reduce draw on the battery bank.
my opinion only
bob g