A fan in a window or roof vent works best when it has a shroud so that the air moved cannot make a 180degree turn, and run back through the window only to be sucked in by the fan again.
Positive air displacement via a tight fitting shroud works to keep a rig cooler.
I make widespread use of computer fans, but to move a lot of air, these generally will be loud.
Speed control is very important to me, and I use the 'Loud' for when it is just searing hot, or I need to drown out screaming kids or barking dogs, ect.
These two computer fans come with built in manual speed control:
https://www.amazon.com/SilverStone-FM121-Control-Designs-120X120X25mm/dp/B000BWEIFU
https://www.amazon.com/SilverStone-...=1468828280&sr=1-1&keywords=silverstone+fm181
This 180Mm silverstone fan is very quiet even on highest speed, moves a good amount of volume on highest speed (~150cfm) at ~0.28 amps and is practically silent at lowest speed, 0.08amps and ~50cfm.
This following 180MM fan is far more powerful, but it does not like running at its highest speeds at battery charging voltages. Mine failed, but salt air corrosion was a factor along with a few times early on exposed to 14+ volts at max rpm and the hub got stinky plastic hot.
https://www.amazon.com/Silverstone-...=1468828468&sr=1-1&keywords=silverstone+ap182
I keep one 180mm fan and 2 120MM fans individually switched and speed controlled in a conversion van window with a shroud. Rare that I need all three running full speed but much appreciated when i do.
Other computer fans can be speed controlled with a PWM motor speed controller. This controller should have a 19 or higher Khz frequency, otherwise the PWM signal makes the fan whine annoyingly.
21 KHZ but 10 amps is overkill on most fans. Unless one perhaps wants to go for a smaller radiator fan.
https://www.amazon.com/Controller-A...ontroller&qid=1465288414&ref_=sr_1_35&sr=8-35
Some other computer fans are called PWM fans, these have 4 wires and one of these requires a PWM signal be fed to it to control the speed. The other wires are + and - and a speed sensor to feed to a computer motherboard. it is said it that it is better NOT to control a PWM fan with a PWM motor speed controller on the + and -wires but to use the PWM wire.
I will soon be utilizing this 4th wire on a Noctua Industrial IP52 rated 3000 rpm 120mm fan, and perhaps also be able to tame the screaming banshee fan to tolerable levels. This NOctua has nearly double the static pressure rating as the 120MM silverstone but about the same CFM rating, meaning it will be good at forcing air through resistance better than the SS fan.
This 120Mm fan is insanely powerful, and LOUD, so I call it the screaming banshee:
https://www.amazon.com/Original-Del..._SR160,160_&psc=1&refRID=G9AY74H1485VEC1PDDVM
I don't think this fan, if it can be tamed to a quiet enough level for low airflow, would be very efficient, electrically, but turning that dial to high could exhaust all the air in my van Approximately every 2 minutes.
the screaming banshee only slows to about 60% speed before shutting off on a PWM motor speed controller, so i hope to feed the PWm wire separately, and achieve slower speeds.
My interior fans are smaller Vantec Tornado fans in 80 and 92MM sizes. These are very powerful fans too. i have them on a gooseneck attached to a 2 inch spring clamp, so I can mount and aim them anywhere.
https://www.amazon.com/Vantec-TD923...=1468829392&sr=1-1&keywords=vantec+tornado+92
To control the speed on these Vantec tornado fans, I use a ciggy plug style voltage controller.
One very similar to the one below. These fans fed only 3volts are very quiet and draw very little juice. However they might not start turning on 3v when first plugged in, so start at a higher voltage then back off. These fans send a dense narrow column of air a good distance.
https://www.amazon.com/CyberPower-C..._SR160,160_&psc=1&refRID=EDPMM0VY1NHRCD4XBNDN
I am hesitant to post the above link. the plug design is faulty and in some 12v ciggy receptacles ,can be tweeked sideways shortig + and -, and blow the fuse to the ciggy 12v receptacle. I solved this with appropriate sized heatshrink covering all but the spring loaded 'nipple' center (+) contact and these now have thousands of hours on them since purchase.
These two internal Vantec tornado fans are basically the only time I use the 7 different ciggy plug receptacles I installed all overmy rig before I knew better, but as these fans pull only one amp their high resistance and intermittent nature poses little issue in this application, and are convenient.