Glad you have had that luck but you don't live in Arizona and in our constant summer heat. I am speaking of water left in a stored RV. In your case if you are using it at all and adding as you go, you get the benefit of chlorine with new water. If you leave water in any container here, closed or otherwise it will grow nasties. However, even if you are lucky and nothing grows in it that will kill you, most people say that the water begins to take on a plastic smell (and taste if you drink it)if stored too long. In research that I did, most say 4 weeks in an RV tank is about the max that you should store water if you want "fresh".
I left water in my hot water heater, by accident, when I stored the trailer. I pulled the trailer out of storage about two months later (summer) and when I drained the water it came out black and smelly. Granted that could have been from the anode rod but had I not drained it that would have traveled on through the hot system to other parts of the trailer. I really see little reason to "store" water in RV tanks while the RV is out of service. Water is not in short supply, at least right now, in Arizona anyway.
It really comes down to what the individual is comfortable with. If you want to store it that long and are not worried about it, go for it.