2oldman wrote:
Sounds like the tank needs a thorough cleaning.. but I don't know how that's done.
There is a national franchise operation called
All Pro Water Flow that cleans both gray and black tanks with a pressure washer and specially designed small nozzles. We had ours down several years ago when we were experiencing what the OP has reported in this thread. It fixed us up, although we do need to have it done again.
Those of us who full-time in our RVs and have washer/dryers really can't keep the gray tank valve closed all the time unless we wish to risk washing machine overflow accidents. I don't mind having the tanks cleaned every couple of years as the price I pay for this convenience.