Make sure your tanks are full or close to it (both Black and Grey) then dump black first followed by grey. Dumping full tanks causes a lot more force of water "pushing" out the chunks.
I also use the Flush King. This allows me to close the valve and fill the tank back up to rinse. The clear elbow allows me to see when tank is "clean" and I can then rinse the hose as the final step.
Check it out on the camping world website but before you buy do a search for best price.
Flush King