Leave the black water tank valve closed and wait to dump the tank until it is at least two thirds full. You want the tank nearly full so that weight and gravity will force the contents out of the tank.
If the tank is not 2/3rds full when you’re ready to dump then add water to the tank.
When the tank is empty, close the valve, add water to the tank… enough to cover the bottom, and treat with holding tank chemicals to assist in controlling odors.
Get more information on RV holding tanks at RVbasics.com
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