you have a 1000litre tank already, you just need an ro system, i run my 450gpd from my kitchen sink and into the 1000litre hoilding tank, then after the ro you can put a di polishing unit at about 30 quid or you could use the one in the van.
if you live in a hard water you will use your resin up very quickly a £70 bag will last you a couple of months using an di vessel only however with an rosystem, it will last you up to a year, variable on both counts due to how hard your water is.
also if you bought a booster pump for your ro system your resin will last even longer, becasue the ro will be working as it should under good pressure.
in my opinion if you ran only a di vessel you will very quickly get tired of buying resin all the time and buy an ro system anyway so save the money in the long run