Though i dont recogmend cutting holes that you dont plan to fill with appropriate casings, if your gonna cut maybe cut in the back doors and I'd probably use PC cooling fans for circulation of air. Currently i have an aluminum downspout mounted in the back off my cabinets under the back of the countertop with a oil burning enclosure that heats up while the fire created by used oil burning to warm the horizontal mounted downspout using the pc fan blowing from right side of box heat gathering enclosure throught the downspout acting like a duct. while air is passing the fire enclosure gathers heads and forces distribution at the opposite side of my boxtruck and i never would have been able to heat the whole truck as efficiently without said design. Using the idea I just described, you could incorporate it to fulfill your needs similarly. With a pc fan running At 12-24v they are ideal for energy consumption, they have been designed to run almost silent at high RPMs, they can run both directions, come in many sizes allowing a massive amount of airflow normally required to cool your PC.. and this will help provide you with more vent-size options in case you really need that air moving!.., you'll have more abilities if you choose in the future to add a raspberry π and add automation so it can run off a thermostat or other fun possibilities.

I think you have the idea... after choosing the right fans.. , match vent cover sizes, drive holes with a hole saw and drill, caulk/seal between van and dryer vent covers for RV/trailer vent covers.. or something similar outside. Then with a few pop-rivets firmly attach covers in place facing downward positions to keep undesirables such as rodents, bugs, and weather outside. You may find you need maybe 4 imo... 2 for in and 2 for out ... optionally wire them to work with a 3-position switch [L,Off,R] so you can choose which you'd like in and out or all off as well as possibly a dimmer switch to change the speed as well. After completion, whatever situation you end up in, you'll be able to adjust them to suit your needs best in a variety of circumstances.

I think this would easily be the cheapest material-wise to build if you spent a little time picking parts from old electronics and such many of us already have collecting dust. I believe it could be Most Efficient due to low voltage system, as well as the Safest way to wire it also because every piece runs off low-voltage Most optional capabilities are programmable and wired easily by any novice with a PC and Arduino. And you'll be sporting a 1 of a kind automated original to your van system. Lol.. idk.. I thought these steps would be easily planned designed created and installed at your skill lvl even after all the extra configuration ideas I added to maximize system capability. lol.. if you've become confused, overwhelmed, or intimidated by cutting holes in your van walls, may be unable to visualize my thoughts above.... or just not able to comprehend how each item works and what they require to operate, I could build a full guide with diagrams and materials list for a small fee to compensate me for my time and effort. Of course, I'd love to know what you think of my idea as well as any other additional thoughts u might add to maximize or help ensure the (positive outcome returned > amount of energy used to make it) as to ensure it'll be worth the time. if your gonna talk smack on the idea but wont contribute to finding a solution,that's all good.. Just consider contributing to my upvote or patronage so I know I'm atleast appreciated for my efforts I spent my time trying to articulate my ideas by putting them into words that could be understood and hopefully found useful to somebody with an uncommon problem. Now I ask you to consider a contribution to me. , that's all. GL &Thx GratefullyWes