What's in the tank? I hope not sewage.
Saltwater and urince cause calcification in the sanitation system. This will clog pipes and seize valves. A coat hanger may not work. What does work is Muriatic acid. This will dissolve the calcification and allow the valve to open or unclog the pump. I had this happen with my direct discharge through hull. I injected some Sew Clean into the hose and the valve loosened up and worked. It did take a few to work.
When I bought the boat the Y-Valve was almost completely calcified. I soaked in a bucket of Muriatic Acid and it cleaned up nicely.
View attachment 231582
View attachment 231581
Cleaning a calcified Y-valve with 30% muriatic acid. The reaction is quick!
youtube.com
Sew Clean® is the best and most proven method of removing any clogs, oders, and build up of uric scale in black water systems.
www.trac-online.com