If you do not want a powered option Fantastic Fan makes a static vent that does not require power. It has a manual crank to open it. We have one and it works great to add cross ventilation with a window open..
Read detailed trip reports, see photos and videos on my travel blog, luinil.com.
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Current van: 2002 Ford E350 extended body camper with Colorado Camper Van pop top and Agile Offroad 4WD conversion.