It boils down to the amount of air the fan can push/pull depends on the area it's trying to push/pull the air through. Kinda like a water hose diameter, 5/8" hose will give you more water per minute than 1/2" hose.
We have a MaxxAir II cover over 1 high speed fan and a FanMate cover over another high speed fan and I can tell very little difference in air movement. So IMHO the MaxxAir II should work just fine for your application. Have you seen these new ones? http://www.campingworld.com/browse/p...847925&rewrote
Hard to tell from the pic but looks like they might allow the fan cover to open further than the MaxxAir II. But they are currently $10 more that MaxxAir in white.