I have four rainwater tanks, coming off of four different roof sections. Could use a couple more. Only one of them we use for drinking water. The rest are great for the garden.
So I refill the potable tank in mid winter. The roof is clean, so are the pipes. I can pour out what's already in there and allow a huge downpour to put fresh water in there. Only needs to happen once a year. I think what I'll do for a little while is to let it pour out from the bottom into the garden, even while it's raining. So there will be no stale water in there.
I know it seems obsessive but these things - water, food safety etc are important. Imho.