How long do you store fresh water in tank?

When connected to our municipal water here when we are not traveling, I always keep the fresh water tank completely full. We never know when we may need fresh water during hurricane season. If we sit the storm out or if we have to evacuate, we want to leave with a full tank. In several decades of RV travel, we have never had our water go bad to the point that it either stunk, tasted bad, or made us sick. We do have the under sink filter for any drinking or cooking water.
I would worry about an empty tank, that is NOT completely drained(how can you tell you drained every drop) that is full of air and has a better chance of breeding mold than a completely full tank of chlorinated water.