You can make a computer fan variable speed by simply varying the voltage, OR PWM switching pulses. It is probably not worth the bother as a computer fan is pretty weak for ventilating a large area, but I could see reducing power to it, to reduce power consumption from a battery.
What you can't do with a brushless DC computer fan, is reverse the air flow. Instead you'd want a brushed fan for that, then you simply reverse the polarity. To clarify other motor types can reverse polarity too, but with a DC fan, and I assume you want low cost, brushed would be it.
There are many purposes/sources for such a fan but the most obvious that comes to mind would be a radiator fan for a motorcycle, or secondary radiator fan for a smaller car, or of course an RV vent fan. It will have far better performance in reverse mode, if the blades are fairly flat instead of curved but then it will also have worse performance per watt, in its intended primary flow direction.
Frankly I don't think you really need it reversible, only to pick which way it flows with considerations of whether it will be on while towing the trailer /camper and if so, then consideration for operating in same direction as the wind, or not sucking in water if it's raining but then, the typical computer fan is not waterproof, but the major fan manufacturers do make 12VDC, waterproof fans... for a higher price of course. Any of the motorcycle or other radiator type fans will at least be highly water resistant.