Here is what I just read on RVbasics.com web site. Don't know if its right or wrong you decide.
Generally Accepted Method to Sanitize Fresh Water
The generally accepted method of sanitizing your RV's fresh water system as outlined below involves filling the fresh water tank with a solution of household bleach and running the solution through each faucet. Then letting it stand for at least three hours. Finally, flush the system once or twice to remove the taste and smell.
This procedure is one you'll find in most any book about RVing it tried and true but be sure to read on to find out what I do.
Start with a nearly full fresh water tank.
Turn the water heater off and let the water cool.
Dilute 1/4 cup of household bleach for each 15 gallons of tank capacity in to a gallon of water.
Add the chlorine/water solution to the water tank. (Never pour straight bleach into the RV fresh water tank. )
One faucet at a time, let the chlorinated water run through them for one or two minutes. You should be able to smell the chlorine. (Make sure you are using the water pump and not an external water supply.)
Top off the RV fresh water tank and let stand for at least three hours over night is better.
Completely drain the system by flushing the faucets for several minutes each. Open the fresh water tank drain valve to speed up emptying the tank.and Open the hot water tank drain plug and drain until it is empty.
Close all valves and faucets and drain plugs.
Fill water tank with fresh water.
Flush each faucet for several minutes each repeating until the tank is again empty. (Make sure you are using the water pump and not an external water supply.)
Fill the tank again. The water should now be safe to drink but if the chlorine odor is too strong you can repeat the fresh water flush.
Your RV fresh water system should now be safe for use.
The way I Sanitize My RV's Fresh Water Tank.
A friend and fellow full time RVer, Bill Randolph told me how he uses something other than household bleach which works well for him and I also started using it. I think it is worth passing on to you. Before he retired, Bill spent twenty years in the swimming pool business. He's an expert when it comes to sanitizing swimming pools and spas and says the same rules apply to RV fresh water systems.
Bill uses Chlorinating Concentrate (Sodium Dichloro-s-Triazinetricone or Sodium Dichlor for short). Sodium Dichlor contains 62% available chlorine. Compare that to household bleach which has something close to 3%. One pound of Sodium Dichlor is equal to 8 gallons of bleach! Also, household bleach contains other stuff, including a lot of salt, and that salt and other stuff is what causes the bad taste and why you have to flush the fresh water tank so well.
Bill says it takes only 1 teaspoon of the concentrate per 100 gallons of water to initially sanitize the system. Remember to run water through all the faucets. It's okay to use the full teaspoon even on smaller tanks because you will be flushing the tank before adding the water you intend to drink but it seems wasteful.
Like most of us, Bill travels with a near empty tank to reduce weight. So if he arrives at park where he plans to stay and they have well water, he drops a half teaspoon per 100 gallons of the concentrate into the fill tube and fills his water tank. This insures the system will always be sanitized. No, you do not have to flush again. It's the equivalent to drinking chlorinated city water. If you are filling your tank from a source that is already chlorinated then you don't need to add the concentrate.
That said, if you don't like to drink chlorinated water, don't add the concentrate to the water you intend to drink. Assuming, you fill your water tank from a trusted source you should be safe. Or, you can add the concentrate and then filter the water you drink or cook with.
Truth is I almost always seem to be filling the fresh water tank from a chlorinated source (city water supply) so I seldom need to to use the concentrate. And we do filter our drinking water.
This is not a case where more is better. This stuff is concentrated and it's best to use just what Bill recommends.
Because Sodium Dichlor is so highly concentrated you only need to carry a very small container... buy the smallest container available. And it is dry crystals so there is less chance of a spill. However, because it is so concentrated it is highly corrosive so you do have to be careful how you store it and use it. You should be able to find Sodium Dichlor (Chlorinating Concentrate) at any pool supplies or spa store. Bill says there are several brands to choose from but brand should not be a factor in your choice... it's all the same stuff.
You may also want to read: RV Fresh Water System Care and Maintenance
and RV Fresh Water System Accessories