Allow waste in the black water tank, which contains waste from the toilet, to accumulate until the tank is about two thirds full. According to RV Basics, adding water to the black water tank allows the RV owner to reach this level whenever they want to drain the black water tank.
Attach the drain outlet of the RV to the septic system. Most RVs have a flexible hose storage area in the back bumper. This hose connects to the outlet of the RV and into the septic system opening. Open the valve for the gray water, the storage tank for waste water from the sink and shower, and allow it to remain open while connected to the septic system.
Drain the black water tank by opening its control valve near the gray water valve. Allow the black water to drain, and flush the tank and hose by running water through the toilet.
Close the black water valve. Run enough water into the black water storage tank to cover the bottom of the tank and add RV sewage tank chemical to the tank.