I have just gone through the same thing with my system. The company that was the supplier to Winnie on the system was Swan Industries out of Bend, OR. They no longer use the same valves on their systems but when I called them, they were very helpful and sent me a couple of the diaphrams that came off used valves. I took mine out of the coach and performed the repair on the bench. Lasted for a 5 day trip to Mexico and when we got back, I noticed that we have a leak again. I have not determined if the leak is in fact the valve or a connecting fitting that I had some trouble with when I put it all back together. Will be taking a look at it in a couple of weeks. I would definetly try talking to the folks at Swan bvefore I would pay almost $3000 for a new valve from Winnie. When I was checking on replacing the valve, my local dealer gave me a price of $189 for the valve. Still overpriced but a bit better than they are quoting you. My next step would be bypassing the valve entirely or replacing the system if I needed to winterize (which I do not).
Tomcat 15 (Rich) has a history with these valves as well. Give me a email at firstname.lastname@example.org
and i'll give you his email. Don't want to put it public.