Signed up again, to give ya a thought. I think your vent is blocked, or your vent pipe is too low in the tank. Air admittance valve? Maybe, but I don't think so? My vent to roof for my galley tank, became partially plugged once, which caused that tank to drain really slow. I removed roof cap, run garden hose down, using water pressure, which opened it up, from the obvious bug nesting, and has drained normal ever since.
Does your shower/sink tank drain normal speed, when you open the dump valve?
Hope you get problem solved, have a great day!
Jerry