You can try one of these.
https://www.amazon.com/dp/B072VSRG8T...NsaWNrPXRydWU=
I was suspicious that it was just another gimmick. But tried it and was surprised.
After a normal dump and after using the Black tanks internal rinse system. I hooked up a hose and flushed it for about 5 minutes. Nothing dramatic happened until I shut the water off. A considerable amount of "stuff" came out. So it definitely helped.
I would imagine it probably depends on how convoluted your piping may be from the connection. Mine is a straight shot into the tank.
Good luck.