Instead of putting your RO/DI in the van, you can get a 1000 ltr IBC and fill your pure water into that. You can then transfer this water into your van by using a submersible pump. I did this, so it's easy to do.
Also, you'll need for inside your van:
Pump
Flowcontroller
Battery
Hose,
Hosereel
If you're doing a two man system, I recommend two pumps and two flow controllers, both of which you can run from one battery.
I use a leisure battery, but a lot of guys here use diesel vehicle batteries they get for a tenner from a garage.
If you can be bothered, have a look at my backposts at about March time; that's when I went van mounted and asked a boat load of questions.