go for a 300 gallon r/osomos system(all the companies mentioned on here sell them) fit it with an ibc plastic holding tank into yr garage or shed,then when the water has been filtered and is pure you will have to pump it into yr tank in yr van,or into containers if you are using a trolly system.
If this system doesn,t supply you with enough water have a look at the merlin system,which i haven,t a clue about.....someone may beable to explain it to you in layman's terms.
also if your tap water reading is low some lads just fill up their tanks in their van with tap water, the water then goes through a d.i vessel and comes out pure at brush point ..again someone else may be able to explain it better as that is just my general understanding of it.
all the best with yr journey shiney!!