When we bought our 1990 Winnebago in 2007 or so, I sanitized the tank. I noticed that there is a 1/4" clear rubber tube that goes from the tank to the fill nozzle as a vent line so that as the water goes in, air can get out. It also lets the water level decrease in your tank without causing a vacuum.
That air vent line was full of green stuff. So I removed it and ran a bottle brush through it and put it back together.
Problem is this vent tube usually does not get water in it until the tank is full.
But we never use the RV water for drinking or cooking. We always use bottled water for that. RV water is only for bathing or teeth brushing or washing dishes.
So I have not sanitized the water system since. It gets filled and drained every trip, about once a month.
Maybe this spring I will do the sanitize thing again.