By no means an expert on plumbing, but what about an actual electric water heater...heating element removed/disabled... plumbed in series with the working water heater/s so that cold intake goes to the dummy...then to the working heaters. That way as you use hot water in daily life the "water storage" dummy heater's water drains/refills and stays reasonably fresh. Ball valves and drain valves could isolate this storage drum.
IF....and I don't really know...there is a reason why you shouldn't consider the water in the actual water heaters potable.