On our trailer, one of the valve handles - I think the black water - does not have a straight pull on the valve, the operating rod has a double ninety degree bend behind the bracket that is just behind the handle so as to offset the pull. This makes it much harder to pull the valve using the handle.
I generally reach in behind and grab the rod at a point where it allows me to get a straight pull on the valve and then it works fine. Not a good design IMHO.
Just wondering if that might be the problem that you are having.
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.