I put one on our 23B and it was open 24/7 for roughly 2 years. I don't recall any water getting past it. It was the original MaxxAir. When we sold it and got the 5er I installed the MaxxAir II and it has proven just as dry. However in hot, humid weather I now close the vent to avoid condensation on the ceiling where the ducted A/C channel runs.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks