Like moorefc, I do nothing but use chlorinated city water, and have never had a problem in 10 years. It is rare for a couple weeks to go by without using our van, so I just always add new water to the old. We have twin 16 gallons tanks, so a big capacity. I do drain it each winter to avoid freezing for a few months.
We almost always boondock, but one time in an Arizona state park we got a bad batch of water from our campsite source. After green gooey chunks
started coming out of our faucet we did head for a town for a bleach purge!