I was reminded of something else that happened to our 166FBS that may be affecting yours.
I was draining tanks after camping in preparation to return home. As usual, I was flushing the stinky slinky when a fairly large, round piece of plastic came out. Turns out is was the circle that remains when a hole saw is used to cut a hole in the plastic holding tanks. Unfortunately, I didn't know which tank it came from.
Perhaps you have a similar situation, only the plastic circle may be stuck sideways in the drain, slowing down the flow. The only thing that I can think of to free it up is to run a hose up thru the outside drain pipe and try and flush it loose.
Just a thought.
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