How can I safely clean and disinfect my fresh water holding tank?

The following is provided courtesy of Everpure, Inc. from their booklet
What Every RV Owner Should Know About Water

HOW CAN SLIME, ALGAE, OILY TASTES, ODORS, ETC. BE REMOVED FROM A USED WATER SYSTEM?

First, the system must be cleaned to get rid of clinging materials and sediment. To remove clinging oils, films and slime, the entire water system should be flushed using 4 teaspoons of household detergent for every 10 gallons of water. The soapy water should be run at full force through all of the outlets in the vehicle. This means opening the taps in the shower, the lavatory and the sinks. The water tank is then refilled with clear water and the entire system rinsed by running the clear water through all outlets. Merely filling and draining the water tank alone will not do a through job.

NOTE: Be sure to remove any faucet aerators and the shower head during the cleaning procedure. They may be reinstalled before disinfecting if desired.

After the system is relatively clean, it can be disinfected. Six ounces (2/3 of a cup) of liquid chlorine bleach (Clorox, Purex, etc.) should be added for each 10 gallons of water the tank will hold. This makes a very strong (200 parts per million) disinfectant solution as suggested by the United States Environmental Protection Agency. By half filling the tank, then adding the bleach and finally filling the tank to the top, a thorough mixture of chlorine and water is assured. Every water outlet in the vehicle should be individually opened to allow the chlorine solution to run through it until the chlorine smell is noticeable. This procedure is repeated at one hour intervals until the chlorine smell is immediately noticeable.

For badly contaminated water systems this procedure may have to be repeated many times, until the smell of the protective chlorine is evident whenever the water is running.

Next, the water tank is filled with fresh water and the entire system and all the faucets flushed thoroughly until the chlorine smell is gone. The diluted bleach solution rinses out pretty easily.

You can also run a baking soda solution through after the bleach flush to help freshen the system. The baking soda solution helps take care of the rest of the bleach odor and freshens the tank so you don't feel like you're drinking pool water.