Making RO outside in the winter?
No idea how far you are wanting to go, spend or other details so I'll just give you a scenario on what I could execute here with relative ease and possibly work around things you've said so far:
I have a cold water storage tank in my loft as part of my heating system so have water and could quite easily add another for the RO unit to fill on a float switch, below the storage tank is the airing cupboard and I could quite easily drill another hole through the ceiling so RO water from the storage tank was quite literally on tap in the airing cupboard, connect a hose adapter onto the tap and you now have RO at height which could be delivered directly to your aquarium through a hose- If you could get permission for something like this or would work is very much a different matter.
I have a tap adapter on order so hopefully can do it indoors. Unfortunately now have a combi boiler so no storage upstairs, bu got me thinking about installing the RO unit next to boiler!