My big brother is an expert on water and what it may contain. He tells stories I don't want to hear.
The following article is from Royal Doulton, makers of fine china, and, since 1835, ceramic water filters.
Note that this article calls for the use of "liquid soap" to clean the tank. The amount mentioned was too much in my case. It made a LOT of suds, and took forever to wash out.
They are in Canada, so their definition of "liquid soap" may be different from mine, or they tested with water that was much harder than mine. I recommend that you consider your own situation when adding the soap.
How to Clean Your Fresh Water Tank, From Royal Doulton:
"The first step is to clean the tank and line, as this will remove clinging oil, slime and sediment. Add four teaspoons of liquid soap for every ten gallons of water held in your system. Pump the solution through all the outlets and into your holding tanks.
If your RV is mobile, take it for a short drive. The "sloshing" will be an added bonus in the cleaning process. Drain the soap/water solution and refill with clear water. Continue to pump clear water through the system until all evidence of soap has been elliminated. If you have not been performing this act regularly, then a repeat performance is recommended!
Once the system is clean it can be disinfected. Disinfecting does not "clean" the tank, it only kills bacteria that has gained a foothold in your system. Fill the fresh water tank halfway with water. Add 6 ounces of liquid chlorine beach for every ten gallons of water the tank will hold and then fill your holding tank to the top.
Open all water outlets and pump the solution through until you can smell the chlorine in the water coming out. Shut all faucets off and let the system stand for at least one hour. Reopen the faucets and pump the remaining solution into your holding tanks.
The system should then be thoroughly flushed with normal chlorinated tap water until the smell of chlorine has diminished to the same level as that of the water you are using to flush with.
Cleaning and disinfecting should be done at least once a year, or anytime contamination is suspected. This is also especially important before a new tank is used for the first time.
The tank is now ready to be filled for your trip. At this point, the Environmental Protection Agency (EPA) advocates using a method called "Super-chlorination / de- chlorination" to prevent bacterial growth while traveling. By this method, chlorine is added to the water in increased amounts to provide a minimum chlorine residual of 3.0 ppm (parts per million) for a contact period of five minutes.
You now have a reservoir full of water with a high concentration of chlorine. It's safe from a baby boom of bacteria, but it's definitely not the means of a good cup of coffee. This is where the need for a de-chlorination device comes in.
How To Super-Chlorinate Your RV Fresh Water Tank
The chlorination method recommended is to add one teaspoonful of chlorine-type liquid bleach (Clorox, Javex, etc.) for every ten gallons of water your tank will hold.
E.G.30 gallon tank = 3 teaspoons = 1 tablespoon
The most effective method of putting chlorine into your water system is to first connect your hose to your RV and then pour the bleach into the opposite end of the hose, prior to connecting it to the filling source. Use chlorine every time you fill up. This will also keeps the filler hose sanitary and protect it from becoming contaminated. Use a chlorine test kit regularly to determine the residual chlorine level (3.0 ppm recommended). Testing should not be done immediately after filling, wait until the water has been "standing" for at least 6 hours."
Read more here: