[Richard and Rhonda]

Yep, I had exactly the same problem. If am assuming you also have the Dupree valves. If you look at the top of the valve you will see two screws. If you remove them you can remove the slide gate. Mind was all mangled and that why it wouldn't close. I called Dupree and had a new one and a spare in a couple of days. Here is their website http://www.dupreeproducts.com/

Now, let's talk about another comment you made. You said open the air operated dump valve to let the grey out. Mine is not configured that way. My air valve is on the passenger side and I think it drains everything. On the driver side I have two dump valves one that isolates that tank, and one that if will allow grey water to pass directly to the sewer hose.

Yours may be different, but it might pay to trace out the piping to make sure you don't make a oh=oh.