Grey Tank Valve Stuck Closed

It happened to me a few years ago. I dumped the black tank, then washed out the sewer hose well. Then I held the open end of the hose (the end you usually attach to the drain)under the valve while I slowly loosened the screws on the valve until it started to leak. With a little trial and error, you can adjust the leak to a manageable flow by loosening or tightening the screws. It takes a while, but the tank finally drained. I bought a new valve and installed it to correct the problem. I'm just glad it was the gray water not the black. Now I replace the valves anytime they start getting hard to open. Probably overkill, but valves are pretty cheap and it's a whole bunch easier to replace them when the tank is empty!