Buy non-toxic antifreeze specifically designed for RV use. Read the label to determine the amount you'll need for the size of your holding tanks.
Empty the black and gray water holding tanks.
Close the dump valves. Store your hoses until you're ready to dump the tanks. Do not keep the sewer hose hooked up while you are parked at an RV site, as the black water tank needs time to break down solids in liquid, or it eventually may not drain properly. You also cannot empty the gray water tank onto grass or a garden when it contains antifreeze.
Add antifreeze to the black water tank through the toilet. Add antifreeze to the gray water tank through the shower drain. You probably will be adding about 2 quarts to each tank, depending on the size.
Dump the black water tank when it becomes half full. If you must wait beyond this point, add another quart of antifreeze when the tank reaches this level. Otherwise, the antifreeze may become diluted. Add antifreeze to the gray water tank as well if you cannot dump it by the time it reaches half full.
Add another 2 quarts of antifreeze to the tanks after you dump them. Follow the procedure over again.
