Park the RV in a flat and level location where the excess water can be safely dumped from the water tank. Open the fresh water tank's empty valve. The location of this valve varies from RV to RV but is typically found under the RV at the tank's bottom. Consult your RV owner's guide if you cannot locate the valve.
Seal the empty valve. Go to the water tank fill valve and open it. Place the funnel into the valve and add one cup on bleach to every 15 gallons of capacity the tank has (for example, if you have a 30-gallon tank, add two cups of bleach).
Fill the tank two-thirds of the way. Close the valve cap.
Drive the RV for 30 minutes. Find a route that has turns, curves and hills. The purpose is to slosh the bleach water solution around the tank as much as possible to remove the black mildew stains and film.
Return to the RV parking area and open the drain valve. Close it, and then go to the fill cap and fill the water tank two-thirds full again. Drive for five to 10 minutes. Return, drain the tank and fill it again. This removes excess bleach from the sides of the tank, making the tank clean and suitable for water.
