I think it is a good idea to sanitize your water system whether or not you use it. A few years ago, we purchased a motor home that was 8 yrs old at the time and the folks we purchased from had used it for tailgating and only used the fresh water tank to flush the commode.. anyway the water was so stinky it smelled like sulfur. When I tried to sanitize the tank, there was algea growing there and it stopped up the pump filter! I never got into such a mess! I finally got it cleaned out and sanitized (several times) but I could never bring myself to drink the water or use it to cook. That's one lesson I never forgot! We have since purchased a new RV and I sanitize the water system at least once a year,even though we seldom need to use it where we camp.