Sanitize or replace old camper fresh water holding tank?

when you run sanitized water through the lines it contacts all the inside of the water line.
no matter how full you fill your fresh water tank it will not reach the top of the tank.
how do you scrub the upper part of the tank?
will it be sanitized from driving around for the sloshing effect to clean all the tank.
if I were worried I would change the tank.