Like many others, I leave them open when not in use camping. Hot/humid or otherwise, I would think air exchange is a good thing.
One interesting point....and not one I'm sure makes a difference or not....I close the vents while on the move. I'm thinking the less banging around and bumping there is on the hinged vent cover the better. With a rear kitchen unit, one learns about "bumping around" the first trip out.
2015 Freedom Express 246 RKS
2009 GMC Sierra 1500 Z71
Days camping previous years: Not Enough
Days camping in 2016: Still Not Enough