If you have tank sensors, I am sure this has happened to you. You know that the tank is empty, but the sensor says that it is half full. Most likely this is due to residue or something that has gotten stuck to the sensor in the tank.
Here is an easy way to clean out your gray and black water tanks. Before your next trip, drop about a half cup of dishwasher detergent down the toilet and down the sink drain along with a few gallons of water. As you drive to your destination the dishwasher detergent will do what it does best – break up all of the stuff that has accumulated in the tanks. When you get to your destination, use the dump station to drain the tanks and ta-da! They will be squeeky clean!