In 50+ years of camping I have never "cleaned" my H2O tank. Once in a while I might put a cap full of sodium hypochlorite in the tank if I remember. I have never been sick on vacation, and neither has any member of my family.
Maybe I'm a cockroach or something? Maybe I have a great immune system? Maybe the reason I have a great immune system is because I tax it? Maybe I'm lucky? I don't know? Just the way I do it.
Oh, and as far as it's just a holding tank. See those big tanks up on the hills of a city? What do you think they are? :R
Oh and before anybody starts on the bottle water kick with really cool names like "Spring fresh ultra mountain water". Most are LA tap water put into a little plastic bottle and sold for more than gasoline. :B