When camping with hookups; I hook up and turn on the city water. Use it till we leave, and have never turned it off when leavign the RV, although we don't leave for more than a few hours at a time (down at the beachside). Black tank closed, grey open until last day, then close it to build up enough for a good rinse of the hose after dumping the black.
Oh, and...been RVing for 26 years and never "sanitized" my fresh water tank. My city's water has a bit of chlorine (as do most), and we use the RV typically every other month...still alive, water still comes out fresh, used for washing, flushing, cooking, drinking...no problem.