The chlorine in city water is only effective for a short while. If the water sits for very long it will grow organisms. In very rainy times when much less water gets used because no one is watering their lawns, cities can have problems with their water tanks going bad from low usage. The RV systems need to either keep treated water flowing through at a good rate (refilling and using up and refilling) or be emptied and sanitized periodically. Sanitize by emptying and refilling with 1/4 cup bleach per 10 gallons of water, open all the faucets until you smell the bleach, let it sit for a few hours or more, then empty and rinse until the smell is gone. I close off the hot water heater from this process but make sure I open it and get it thoroughly rinsed with fresh treated city water.