When I bought my Winnebago, the manual recommended the following.
FOR EACH 15 GALLONS OF CAPACITY (I have a 40 gallon tank):
1/4 cup of bleach diluted into 1 gallon of water poured into the fresh water tank. Fill the tank with water. (If you don't have a gravity fill, pour the undiluted bleach into the fill hose then hook up to the water.)
Drive for a few minutes to mix the bleach/water and to slosh it onto the top of the tank.
Run every faucet/shower/toilet until you can smell the bleach odor.
Let it sit 4 hours, then drain and rinse with fresh water.
If you double the bleach, you can cut the wait time down to 1 hour.
So, since I have a 40 gallon system, I use 3 of those for a total of 3/4 cup of bleach with a 4 hour wait.
