Take the R.O off and put it in your garage as a static set up...?
Cant you put a split feed on your tank outlet (or after your pump)with a tap?You could then fill barrels using your van system...
Depends how much water you want really, and water pressure...Varying places say a 150gpd will do you 560litres but that is with optimum water pressure and with your R.O (and booster pump if you need one) on constantlyI would go for at least a 300gpd (https://collinswaterproducts.co.uk/product_info.php?cPath=111&products_id=151&osCsid=ae47a21b0f61291f124a488259afd20b) or even a 450gpd...You'll know for definite that you'll have enough water in your IBC and if you need a booster you wont need it on all the time (less chance of pump burn out and saves on the old electricity!)If you're just using the IBC for barrels, put the IBC on 2/3 pallets and put a connector on the tap (http://www.ebay.co.uk/itm/like/280491247936?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&device=m&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=108) , that way you can just fill your barrels using gravity rather than needing a sub pump