A lot of the answers would be personal opinion and would vary according to circumstances.
As I have nowhere suitable at home for making pure water, I rent a place to put a tank in (I actually do a bit of free window cleaning in lieu of rent). Making pure water in the van is fine if
a) you have a reliable, secure place to park at or near home with access to a tap which has reasonable water pressure and some drainage
b) you don't mind your van being tied up for hours at a time when not working (which may well be the case if you have access to another vehicle)
Even if I did have a) and b), I think I would still prefer a static unit because at least you have some pure water already made up if things go wrong.