AVION Travel Trailers

Does anyone have a surefire method to purify a fresh water tank? There appeared to be some black residue in the tank when I removed it in 2017. I swished a water-chlorine mix in the tank and drained it.

We have never used the tank or on-board pump since then; shore water was always available. That's going to change when we start living in our trailer for about a month while our new house is being completed.

Suggestions anyone?

When I do our water tank, I fill the tank full then add bleach.... I would have to look at the recommendation... I think it is 1/4 cup per 15 gal capacity. I let it sit for a day, drain, fill, drain..... good to go. (this is per the owners manual)

Thank you!