Eyeland:
I had the very same problem you have and it got so bad that I once pulled the seals loose trying to open the valve to the gray water tank. I now know what I did to cause the problem. One thing I did wrong was to put bleach in the tanks to get rid of odors. This causes the seals to degrade. The other thing I did wrong was to use WD40, vaseline and plumbers grease at separate times in a effort to free up the slide. Wrong thing to do! All of the above lubricants are petroleum based and will cause the seals to swell up and make it very difficult to move the blades. I even replaced the entire valve assemblies twice with the same results.
I've since bought new valves again and relocated them so that they are accessible whereas before, they were tucked under and in the middle of the vehicle. I now know to use silicone grease (a non petroleum product) on the blade portion of the valves and have not had a problem since. This stuff is a little harder to find and pricier than plumbers grease but I know HD has it and it can be found online.
