looking for a little roof hatch love

[kyletheyeti]

....
so i'm looking to install a roof escape/vent hatch (one from a school bus) on my skoolie. i'm installing the hatch for roof access and ventilation. i've seen some pictures of the ones with built in 12v fans (these seem to be a lot less common). I talked to one guy who says the reason they're so few and far between is because they're mostly worthless fans. if i could pull air up and out, especially while the hatch is closed, that would be awesome.

has anyone ever seen one in action? i'm curious how they work and also how they move air up through the hatch without letting water in.

thanks in advance for any info!
kyle