My solution to that is what we call my s**t stick. My horseshoe pin got used as a stir stick. Push carefully thru the clog to the bottom of the tank and stir! Keep water flowing and keep up the stir until it moves away. Then you can fill the black tank to near capacity and drain.
2001 Ford F-350 Super Duty CC Dually, 7.3 Diesel
2013 832IKBS Flagstaff Classic Ultra Lite, Hensley Arrow Hitch, Carlisle tires
2016 days camped....211, it ain't over yet!
On the road to ALASKA May 6, home Sept. 13, 2016. 23 states, 3 Canadian Provinces, up to the Arctic Circle, 17,300 miles, 131 days on the road. 1 flat tire, 4 oil changes. What a beautiful trip!