I originally bought a cheaper vent fan on sale at eTrailer.com, like this one:

It was ok as a vent but the fan was noisy and never moved any air since it only covered about a quarter of the total opening.
After two seasons ripped it out and replaced it with a Fantastic Fan model 5000RBT. What a difference! Quieter, moves the air better, more adjustable, better engineered and just built better.

The model I have is reversible, 3 speeds, and has a thermostat. Even at low speeds for a compact teardrop it was so efficient that I felt like I was in a wind tunnel. With exhausting the air the window curtains would raise up. I replaced the 3 speed switch with an adjustable rheostat that allows infinitely adjusting the speed from very slow up to warp speed. Priceless...
I should have just bought the Fantastic Fan in the first place and I would have saved much time and money.
I'm sold on them...