I use city chlorinated water in my fresh tank. I leave the water in the fresh tank between outings, rather than leave the tank empty. If more than a couple weeks before using again, I drain old, and refill with new. IMO, a mostly empty fresh tank, sitting in the summer heat, is more likely to promote bacteria.
In the event I did fill from a source I was unsure of, I'd empty right away when returning home, and add back some city chlorinated water. I would then drain again before leaving for next campout, and refill with fresh.
Jerry