i once helped a guy set up a system in a flat
he fitted the RO under his kitchen sink and ran the 6mm pipe around the top of the skirting into a tank in a cupboard ( he got a 400 L upright tank ) it fitted and worked well enough
ive also on the DIY WFP forum advised another guy, he fitted his RO in the bathroom ( fitted it at the end of the bath and built a cupboard around it, he then ran the 6mm pipe though the wall into the bedroom and in the bottom of his wardrobe
one of them ran a hose pipe ( with a tap on the end to stop the flow ) out of the window and in the van ( gravity did the filling ), the other 1 just used 25 L barrels and would fill them every day ( he made a drip try to catch any spills when filling )
also come to thing of it, another guy on the DIY forum fitted a RO under his kitchen sink and ran the 6mm pipe down the wall into 2 X 210 L rainwater butts right under his window
it can be done, even from a flat