I don't have any hard and fast rule, but if it's been more than a couple of months I just pull the drain valve and refill. Also I don't fill the tank until shortly before the trip, so whatever is in there is going to get a shot of chlorine with the new water.
Less than a couple of months, I just top it off an go.
As far as leaching chems, the plastic in water bottles versus the plastic tank, hoses, fittings? What makes us think one is any better than the other? We keep flats of bottles for years at a time without any concern or noticeable taste change. The difference with a fresh water RV tank is a bit of air flow in/out, and the longer that sits, the more likely that taste may be altered.
BTW, as far as the chlorine in the water, it is dissipated within hours or days. It has already done its job and from that point isn't going to be a factor.