storing in a dark sealed container is fine but if you had a pump moving it around to aerate the ro water that would give you the perfect water to add your salt to.
But as it will be used for top offs lets say 3ltrs a day etc that will be fine i used to store a 5gallon SEALED BARROW of ro water for around 3weeks before i got my ro unit & that did not give my fish corals any problems so you have no worrys.......