Its a vent, all right-Works by allowing air into the tanks when draining, just like your house plumbing vents, which don't have caps on them. Caps on RV vents are more for keeping wasps and or other critters out of the vent than from keeping rain out, caps or no, doesn't really matter until a mud wasp builds a nest in yours and you can't empty your tanks.
Lots of vent caps out there, I've found the cheap plastic caps get eaten up by the sun pretty quickly, so, on my former 5ver, I bought the galvanized metal caps, primed them with gal primer, then a coat of white paint. Stuck 'em down good with Dicor. Good as new when we traded the 5ver for our MH.
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Bill, Kathi and Zorro; '05 Beaver Patriot Thunder
2012 Sunnybrook Harmony 21FBS (SQEZINN)
2007 Jeep Grand Cherokee Overland
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