Roger:
I had the same problem on our DP just after we purchased it. I solved it by a trip to CW. I bought two new Valterra valves (black and grey) and swapped them out. The seals in the orig. ones had hardened (probably due to tank chemicals) and the "blade" that closes the tube was grabbing the mis-formed seals and it would bind in the slot near the bottom.
This blade needs to go past the inner lip of the seal and through until it just passes the outer lip. If you take them off, you'll see what I mean.
The new valves have a slightly slipperier (sp?) rubber on the seals that helps them work smoother. All it takes is a small socket and wrench...or a pair of small crescent wrenches to remove the four bolts and nuts on each of the valves.
BTW, the new valves come with new mounting hardware. Mine were stainless steel as opposed to the old galvanized (rusting) stuff.
This is really a 10 minute job that you can do anywhere...provided that your tanks are empty...
The valves were about $26 for both with the CW member discount.
Ken