I have never replaced one of these blade gaskets in the several different Airstreams I have owned. When mine started letting water go by the seal I just clean them like this;
First fill the bowl with water and let it set a several hours to soften things up.
Get get a wire clothes hanger and cut a piece of about 24". At one end make a short bend about 3/4" long at about a 90 degree angle. At the other end of the wire create a narrow oblong loop for a handle. Gloves on! Open the valve and insert the short bent wire into the groove where the blade runs. Start near the blade and GENTLY scrape away from the blade more than half way around. Then go to the other side and do the same. Repeat a few times more, until no more gunk comes loose. On the opposite side of the opening from the blade when it is open there is a small inward dimple in the circumference of the opening and the blade. That area is usually where the crud collects and the problem usually lies, so give that area a couple extra scrapes.
After you have scraped it several times, flush with water a couple of times to get the loose gunk out. Then flush a few more times while cleaning with a stiff bristle toilet brush between flushes. I like to use an old tooth brush. Be careful to not drop it into the tank!
This step should not be necessary in order to resolve the leak, but I turn the water off and flush to empty the bowl one more time. Once the water stops flowing I add a 1/4 cup of olive oil and let it set there for an hour or so. This will lube the blade and seal.
I do this about ever 2-3 years, or more often if the water starts seeping out.
2014 Silverado 1500 5.3L maximum trailering package (yes, I'm towing the 34')