Gray water tank drain
Does your drain have the HepvO Valve? Mine does and I had a hell of a time getting it unplugged.
I tried the hot water and vinegar/baking soda method, then my Zip-It tool which probably wasn’t good for the valve. Finally I used a length of Cut-N-Tie which is a flexible rubber coated wire that you cut to length and use as a tie wrap. I got that down far enough moving it all around and eventually it cleared.