Originally Posted by Ruger77
What should I do fill with water and try to flush it?
You can try that if tank has not setup too long since your last emptying...most folks on this forum will do a Calgon soap solution in their tank and while driving down the road, it'll help loosen the debris...then empty out once you get home.
Some do a bagged ice method also to help scrub the walls while the camper is in motion...use the search button on this topic & you'll get a ton of info on this subject
Personally, I wouldn't worry much about it...takes a lot longer to fill up the black tank vs the grey tanks on a camping trip.