This is the procedure I use to flush the black-water tank. I have done this every since I was taught by another camper several years back. I have had great results with this method.
I. Insure you have installed a clear water connection to the black water tank. I use a device called the flush king. See link.
http://www.campingworld.com/shopping...ush-king/20522
2. Dump Black water tank.
3. During the dumping process fill two 5 gallon buckets with water. Connect another hose to your flush spray system that is installed for cleaning the tank.
4. When tank is almost empty running slow. Dump one of the filled bucket down toilet, flushing as you dump. Repete process above with the other bucket. You may need to do this three or four times until the black tank runs clear.
5. Turn on the water to the hose installed to flush the tank to the flush system, when running clear shut off. Let tank drain then close valve and continue to drain gray water tanks.
I have never had and odder problems or a plugging issue in the tank doing this method.
Jim