I first found these instructions for recommissioning the fresh water system in 1989. Today you can find them on just about every RV site on the net as well as in both my books:
The following recommendations conform to section 10.8 in the A-1 192 code covering electrical, plumbing, and heating of recreational vehicles. The solution is approved and recommended by competent health officials. It may be used in a new system a used one that has not been used for a period of time, or one that may have been contaminated.
Before beginning, turn off hot water heater at the breaker; do not turn it on again until the entire recommissioning is complete. Icemakers should be left running to allow cleaning out of the water feed line; however the first two buckets of ice—the bucket generated during recommissioning and the first bucketful afterward--should be discarded…bleach does absolutely nothing to improve the flavor of good Scotch!
1. Prepare a chlorine solution using one gallon of water and 1/4 cup (2 oz or 25 ml) Clorox or Purex household bleach (5% sodium Hypochlorite solution ). With tank empty, pour chlorine solution into tank. Use one gallon of solution for each 5 gallons of tank capacity. (Those are the “official” directions. They work out to 1 quart or litre of bleach/50 gallons of water tank capacity , which is MUCH easier to calculate!)
[FONT="]2. Complete filling of tank with fresh water. Open each faucet and drain cock until air has been released and the entire system is filled. Do not turn off the pump; it must remain on to keep the system pressurized and the solution in the lines [/FONT]
[FONT="]3. Allow to stand for at least three hours, but no longer than 24 hours.[/FONT]
[FONT="]4 Drain through every faucet on the boat (and if you haven't done this in a while, it's a good idea to remove any diffusion screens from the faucets, because what's likely to come out will clog them). Fill the tank again with fresh water only, drain again through every faucet on the boat. [/FONT]
[FONT="]5. To remove excess chlorine taste or odor which might remain, prepare a solution of one quart white vinegar to five gallons water and allow this solution to agitate in tank for several days by vessel motion. [/FONT]
[FONT="]6. Drain tank again through every faucet, and flush the lines again by filing the tank 1/4-1/2 full and again flushing with potable water.[/FONT]