The Shiner
If you are in a hard water area and you have not got a RO unit then you can run the tap water though several Di tanks and the resin will last longer and therefore becomes economical, I would recommend 3 DI tanks for very hard areas.
I don't even have a WFP system yet
but I am looking at the feasability of it, what's available, and how I can adapt things around my living circumstances.
What Gaza wrote has interested me for several reasons.
I live in an upstairs flat so installing a holding tank indoors would be impractical due to concerns about leakage into the flat below and the difficulties transferring purified water from tank to van (my flat is a long way from the road).
The difficulties with having an RO van mount would be that the filling up would have to take place quite slowly overnight. The difficulties with this are the lack of access to a tap (particularly for a long period) and also the drainage issues. This would also deprive me of the use of my vehicle if needed.
However, having a van mounted holding tank that could be fast filled with the filtering taking place into a second, smaller van mounted could solve my problems. I could fast fill from a borrowed tap then the filtering could take place overnight within the van. I guess that the smaller tank could have an RO unit as that would not need to fill so quickly. A 250 litre tank could probably supply my daily needs if it is purifying water "on the fly" and if there is a fair bit of pure water in it at the start of the day.
Gaza says he has a 1,000 litre + 250 litre system. I imagine that 750 litre + 250 litre would be enough for me. It would require a hefty payload though and a van upgrade (I only have an Escort 55 currently). I suppose the payload situation could be assisted if you ensure that the larger tank isn't filled unless the pure water tank is empty.
I'm not sure about your suggestion of 3 DI tanks. For the reasons stated above, the holding tank and the pure water tank would all need to be on board. I have considered the possibility of a 375 litre trailer system but I would need to figure out a secure place to leave it overnight while doing the slow fill.
All I know about these issues is what I;ve read on here in the past few weeks. It may be that I've got totally the wrong end of the stick but I would be interested in looking further.
It's unlikely I will be financially able to lay out (borrow) the money to set this up until next Spring but no harm in discussing my options well ahead of time.