Follow these instructions for disinfecting your fresh water tank on a new system, one that has not been used for a period of time, or one that may have become contaminated. This procedure is also recommended before long periods of storage such as during the winter and in fact yearly, no matter how much use.

1. Be sure power to water heater is off. Empty the water heater and screw the plug back in. Do not empty fresh water tank. 
2. Prepare a chlorine solution using 1 US gallon of water and 1/4 cup of household bleach. Pour the chlorine solution into the fresh water tank (a long necked funnel is handy for this). Use 1 US gallon solution for each 15 US gallons of fresh water tank capacity. This procedure will result in a residual chlorine concentration of 50 parts per million (PPM) in the water system, a level that will sanitize the tank.
3. Complete filling the tank with potable water. Shut off the external water source and turn on the water pump. Open each faucet and run the water until a distinct odor of chlorine can be detected in the water being discharged. Do not forget the hot water and exterior shower taps. The water heater holds 6-10 gallons so it will take a bit to refill the tank with the water/bleach solution. Top off the water tank from the external water source so it is full.
4. Allow the system to stand for a minimum of 4 hours to overnight. 
5. Drain the entire system, including the hot water tank. When completely drained, close all valves, faucets and drain plugs.
6. Refill the fresh water tank from the external source. Turn off the external source and using the water pump run water through the entire system to flush, usually 2-3 minutes per faucet, longer for the hot water side. Again completely drain the system and refill with fresh water.
7. Your RV fresh water system should now be safe to use.