There are suggestions on-line. This is the one we follow:
To sanitize the water system use a quarter cup of household bleach for every fifteen gallons of water your fresh water tank holds. Mix the bleach, with water, into a one-gallon container and pour it into the fresh water holding tank. Fill the fresh water tank almost completely full of potable water. Turn the water pump on, open all hot and cold faucets and run the water until you smell the bleach at each faucet. Close the faucets. If it’s possible drive the RV or pull the trailer so the water can move around to assist in cleaning the entire tank. Let it sit for at least 12 hours. Drain the entire system again and re-fill the fresh water tank with potable water. Open all of the faucets and run the water until you no longer smell any bleach. It may be necessary to repeat this process again to eliminate all signs of bleach from the water system. Once this is done it is safe to use your water system.