Having worked on several black tank issues, I have found the best way to flush a black than is to allow it to fill with waste ans chemical to at least 3/4 full and go out and pull the drain handle, when it is through just shut it and step back into the coach put another dose of chemical and add a couple of gallons of water through the toilet. Allow the chemical to do what it is supposed to do, break down waste and deoderize the tank. I have dropped several tanks that needed to be replaced because they were damaged by overfilling with a flushing system. I had one customer run me down on our way out of a park last year after him and a friend returned from a golf game. He remembered that he needed to drain his black tank, after draining the tank, he turned his flusher water hose on (with his drain valve closed). If you check most coaches with a tank flush fitting you will find a label that tells you only use with the drain valve open. His buddy came over with a beverage and the water hose was soon forgotten. He quickly remembered he left it on when he heard a loud "thud". His new fifth wheel had just lost it's black tank. It broke fittings, bent the mounting rail beyond reuse, tore up the underbelly liner and just made a big mess. I had another customer that left his hose connected. He called me early one morning and said his toilet ran over during the night (he had gotten out of bed and when his feet hit the floor they got wet.... When he called and said the water was comming out of his toilet bowl and the pipe was full too, I told him to go and check his flush hose cutoff. He said someone had turned it on just a little bit. The park he was in had several Long term People with kids that was allowed to roam the park. The customer figured one of them saw him and played with his hose cutoff and left it on as a prank. Weather this was what happened or not, it was a mess. After flushing out the black tank several times in a row with a flush system using a water hose, we have dropped a tank for replacement. There is still a "sludge" in the tank. I know there are many ideas about flushing out a tank, just ask anyone how they flush theirs. However, if you just fill the tank to at least 3/4 full, there should be enough vacuum as the liquid leaves the tank to pull most of the "sludge" with it. The rest can be taken care of by chemical and water left in the tank. As most motor coaches have their tanks in the compartments, it is not a great idea to overfill the tank to the point of damage with a water hose attachment. Depending on how the mfg attaches the vent pipe or the fill pipe, you may only have a grommet fitting in the top of the tank. Overfilling the tank may lead to some waste water to run into the compartment. This can only lead to a mess that not many people want to deal with.