I've got a 120mm Antec 3 speed fan mounted in the 14" RV style roof vent in my camper van.
As an exhaust fan, on high, it's bloody good. And not loud, but certainly noticeable. On low, I can't hear it. Even on low, it helps a LOT to keep hot air from building up in the fiberglass 2' tall bubble top.
To move air around inside...not so good. I use a 12v oscillating fan for that instead.
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker