I guess we've just been lucky. We've never cleaned our fresh water tank (if it ain't broke...?). We shocked it with a chlorine treatment once when we first bought the boat. But since we bought the boat, it has been in pretty constant use and the tank has been emptied and refilled with sanitized/treated water regularly. We did install a charcoal filter under the sink and a separate spigot for drinking water and have never had any guests wrinkle their face at the taste -they usually request more because we keep a supply of water on ice in a large pump thermos. Never had any problem with stuff from the tank clogging the system, so hope everything is fine down there. I suppose once we get Libertad back to the states I should put that on my "To Do" list but it's been good for 16 years, so I doubt I'll get into the habit of cleaning it once a year.
In response to the suggestion that a separate tank be used for drinking water, I had a similar concern when I installed our watermaker. I installed a 3 gallon tank in line to the main tank. It acts as a test tank. Every time I fire up the watermaker, if the 3 gallon tank tests fine after several minutes of watermaking, I open up the valve to the main tank. I could never quite get it in my mind that you can change salt water to fresh water and I constantly feared I'd contaminate our fresh water due to a malfunction with the watermaker.