When you are flushing the tank, have someone look under the sink in the bathroom to look for any leaks. The water line comes up through the floor under the sink and goes through a back flow device before exiting back down into the top of the black tank. I had a bad back flow valve and wasn't getting any flow into the tank. A trip to the hardware store for a new back flow valve and parts, it now operates great and I can actually hear the water flushing out the inside of the tank. It works great now. I have heard some other rigs have had the same problem.
The procedure for flushing is what was described in the previous post. I follow the same procedure and also use a flush king to flush the tank as well.