I had the same problem with my dump valves for years and after several sets of new seals and a couple of new valves, I think I figured out the problem. I used to use bleach as a final cleaning step for the gray and black water systems and I'd apply Rectorseal Plumbers grease directly on the valve seals and occasionally spray them with WD40. After a while the valves got harder and harder to operate until they would start leaking and one time, completely came unseated.
I have since learned that one should not use a petroleum based product on a rubber type product because it will swell and soften it. So now I'm using silicone grease exclusively on the seals and no bleach in the tanks and things seem to be working fine. We'll see. I took my first trip recently and so far so good.