1 tablespoon to 1 gallon is the Clorox recommendation. Flush it through all of the lines and let it sit for at least 2 minutes. There are 16 tablespoons in a cup. So if you have a 90 gallon fresh water tank you would need 90 tablespoons or 90/16=5 1/2 cups of bleach.
However, it is not necessary to fill the tank to the top. You could estimate that your tank if half full and add the appropriate ratio of bleach, run it through the lines and then drive the coach for a 1/2 hour or so allowing the bleach to splash all around in the tank. Drain it all, refill with water only, flush the lines and you are done.