If you're using city water, it probably will keep you tanks clean if turned over regularly. Long term storage would be better with dry tanks.
Bacteria can enter the tank in air thru the vent as the tank heats and cools everyday. So even sanitized water, but not treated, will become contaminated. I make all my water, and prior to the tanks the water goes thru a UV light sanitizer. Without treatment, my tanks will become contaminated. From there the plumbing can become contaminated.
I treat my water everytime I make a large batch. The boat holds 450 gallons and I usually let the tanks get down to a 100 gallons before making more. I use a cheap pool test kit, bought on Ebay for $5. I add enough chlorine to just show on the lowest point of the scale. I also use a set of filters before the galley sink that removes any chlorine and odd tastes (if any). Bar sink and ice makers are also on this line. Too much chlorine left in the lines can slowly ruin some rubber parts like valve, pump, and faucet seals.
It is actually easier to stay ahead of water problems than deal with contaminated tanks and plumbing.