Those slits are notorious for clogging with dirt.
Mine were clogging up and water would back up when it rained and water would enter the camper at the windows (inside the walls too).
I opened mine with a Dremel (carbide bit and low speed).
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW