I use a a lot of computer fans.
In cold weather my Silverstone FM121( adjustable speed) on low moves about 33CFM(if unrestricted), is very quiet and I can cook inside.
On high it is rated for 110CFM, and is quite loud. I have a counter rotating Arctic Cool fan rated at 55cfm, inline, feeding the silverstone fan. Inline fans need to be counter rotating. If they both spin in the same direction they both get louder and move less air. Together they are rated for 170CFM or so. I've no real idea what they move together through my Mushroom vent, I'll guess 130 to 145cfm, but the AC fan significantly increases airflow when I switch it on, it also lessens the silverstone fan noise when it is at half speed or more.
From a noise perspective, my ceiling exhaust is not very pleasant on high speed. These fans push air through a mushroom style vent which originally had a 4 inch fan powered by tiny solar panels and had a C type Nicad battery to run it all night long. I've basically gutted the original motor and use only the mushroom housing. Very little lights gets through it. Only if I direct a hose perfectly can water get inside.
Airflow over the Mushroom vent increases airflow through it. Driving at highway speeds with window open and both fans can be turned off and will spin faster than when they are powered when not moving.
A larger fan can turn slower, move more air and consume less electricity.
I own a silverstone AP182 which is a roughly 7 inch fan. On low it moves 65cfm for just 0.05 amps, and is practically silent, on high it moves 170cfm consumes 1.3 amps and is fairly loud, but it is not as annoying a high pitch as smaller fans spinning faster. Its hub gets too hot at full speed when fed battery charging voltages above 13v though. I limit it to no more than 11 volts by a $2.50 gizmo. This does cut the top end speed, but it is so much airflow it is no big deal.
I'd LOVE to have a 7 inch diameter mushroom roof vent and stuff this SS AP182 fan up there as it, at highest speed, is a much more tolerable pitch, and might be able to force air through a restriction better. But the 2 inline fans together are about 0.55 amps and the AP182 is 1.3 amps at highest speed. However at the lowest speed the SS FM121 draws 0.09 amps for 33cfm and the AP182 moves 64CFM for 0.05amps.
The 120MM size is the most common on computer fans. They range extremely widely in quality, amp draw for air moved and noise made.
One of the most impressive companies making computer fans is Noctua. They are not cheap, but they have a 7 year warranty and are among the most efficient fans available. The Noctua NF-f12 moves 53CFM for just 0.05 amps. most Fans in this size consume twice that much or more, to move that much air.
If you want fans in your floor, they might have to be constructed a bit burlier. Noctua has Industrial versions of their regular lineup that move more air and achieve water and dust prevention ratings:
http://www.amazon.com/Focused-NF-F1...36126&sr=8-2&keywords=Noctua+industrial+120mm
I really like the silverstone fans that have their own built in speed controllers, but most computer fans will respond favorable to a PWM motor speed controller, or an LED light dimmer such as this.
http://www.amazon.com/s/ref=nb_sb_s...d+controller&sprefix=pwm+motor+speed+,aps,204
I highly recommend being able to control the speed and noise of a fan through a controller.
http://www.amazon.com/LEDwholesaler...qid=1448336444&sr=8-1&keywords=pwm+led+dimmer
There is the possibility the windings in the motor might make a high pitched humming sound when the speed is lowered via PWM. Some list KHZ rates, and the higher the better in this regard, though Fido might still protest.
If one does not want to punch a hole in their roof at all, A bilge blower fan and some tubing can suck warmer air from the ceiling. The fan itself can be mounted below the vehicle.
But these are pretty loud and definitely need to be tamed down via a speed controller at night. One could employ various dampening methods to reduce their noise, but it might be rather involved to achieve an acceptable noise level.
http://www.amazon.com/SEAFLO--Line-...id=1448336684&sr=8-2&keywords=4"+bilge+blower
http://www.amazon.com/s/ref=nb_sb_s...lexible+duct&sprefix=4+inch+flexible+,aps,202.
I've not tested all fans with all available speed controllers so I can;t say definitively that it will work to ones satisfaction. All I can say is speed control is very important to me, and these PWM dimmers'motor speed controllers, allow one to both change noise, airflow, and amp draw.
I have one super powerful 120MM fan, rated at 252CFM and it cannot be slowed enough to be used in my Van by my PWM controllers. Once reaches a certain speed, it just shuts off.
BUt it would be ideal to have a fan that could move that much air when needed, and also be tamed to whisper quiet too, but I have not found such a product, and am kind of done experimenting at this point as I can turn my Van into a windtunnel and hang out inside in direct sun on hot days, as long as my reflectix is covering the windows.