If your water tank is black it should not grow any algae. They only grow in sunlight, or just ambient light let inside the camper. In plumbing we were taught that all the water in the UK taps contains chlorine specifically to combat 'germs'. I never use any additive to my mh tank. We drink it, wash with it, cook with it and have never suffered any illness as a result. I do drain it every time before we return home, and the driving home sloshes any remaining dregs down (or in AutoSleeper's case where they make water run uphill) to the waste tank.
Putting Milton in it won't do any harm, except to your pocket. Go ahead if you like. If you have a black tank, that is. If you have a transluscent tank, even inside, it would be a good idea.