My mate, years ago, kept marines and used a large water butt in his garage to pre-store his RO water. RO unit was under the plinth in his kitchen and RO outlet pipe ran into his garage. Day before water change, turn on water, put in heater and fill butt. When ready just pumped from butt into tank...job done. Did have butt located outside but had issues with pipes freezing, so was relocated inside.