I put on the MaxAir II on just the bathroom vent and it allows plenty of air out. If you have the Fantastic or other high flow vents, you should really put in a high flow vent cover since they move so much more air.
TV: '06 Ram 1500 SLT QCSB Hemi 4x4
TT: '12 Grey Wolf 27BHKS
Tow Acc: Tekonsha P3 BC, Curt round bar WDH w/sway control, CIPA custom fit tow mirrors