I have an 85 Vanagon Camper with a water tank that was not used for about
the first five years of the vans life, and then was not used for another 5
year period several years later.  Some water has remained in the tank for
nearly a year and it is still cleaner than most of my water bottles.  It's
easily to unscrew the top of the tank, reach into it and clean the inside.
James Wagner