That vent is a good idea that should cut back on the humidity in that space and hopefully prevent mold.
I was thinking you live up north some where so I do have one concern after reading you plan. Is that vent (assuming its at or close to ground level) going to have any issues with snow? Think drifts over the exaust port, or snow being pushed through the fan and into your tank area if the wind is blowing really hard. If you lose power and the fan stops I can see it happening easily, I know people in Iowa who have had snow work into their cars through the heater/ac vents when the car was off. At the very least I would think you would have a very poor ability to keep out the cold down by that fan.
Other then that possible issue. I think it will work out fine.