Thetford's Fresh Water Tank Sanitizer kills more than 99% of bacteria in 60 seconds. It is a non-additive, meant to clean and sanitize the walls of the tank and plumbing, not the water.
Some RVers use chlorine- or bleach-based additives to control bacteria, adding them directly to their fresh water and drinking it. However, bleach combines with organic matter to form known carcinogens called trihalomethanes. And bleach is very corrosive, may damage seals and leaves a lingering, unpleasant taste and smell.