My handles operated my three dump valves by long cables and had got VERY stiff, especially the black tank. I added a small bottle of the valve lube liquid that you can purchase at most any RV store prior to a camping trip once. Seems as if the instructions were to have about 5 gallons of water in the tank, add the lube liquid and let it slosh around while driving down the road. This did help it a lot. I only used the one bottle of this stuff. and that was over a year ago.
Since I started adding Dawn liquid soap to my blank tank after I dump and prior to moving to another campsite, all my valves open and close easily.
2016 F350 CC Dually Powerstroke 4x4
2014 Cedar Creek 34RLSA w/Level Up
2007 HD Ultra Classic 103