You only need ONE system to produce your water, unless you have very soft water indeed, the best route is to go for an R/O system.
A simple and straight forward system is to have a large static tank at your home or place of business, your R/O fills it with pure water, you have a transfer pump to fill up the tank in your van.
In the van you have your tank, a pump, a battery to power the pump, a hosereel and hose and your poles etc.
While you are out working, your R/O is refilling your static tank.
If you have the R/O in your van, then your van is tied up while you refill your tank, and depending on the system you get, that can be for several hours.
Your R/O system is of course a combination of a couple of pre-filters, the reverse osmosis membranes and a DI vessal of some description.
The more you are prepared to spend, the bigger and more powerful system you can get, meaning that you can have something that will fill a 1000l tank in a few hours.
It won't make your production system any better than a cheaper setup. it will just allow you to produce pure water more quickly is all.
What you need depends on the work you do, and whether or not you are a sole trader or work alongside others, either as a partner or your employees.
But get what you NEED not what you want!
For example; I am a sole trader working day in and day out with WFP, on a very busy day, and I mean a REALLY busy day I can use 500+ litres of water, but the average is around 300 litres or so.
I have a 200 gallon a day system, which is just 2 small membranes, and in 3 years I've not left myself short once.
Currently I have also started supplying Squeaky with water, and he will also be very soon using up to 300 litres a day.
Hopefully my small system will supply us both.
I'm sure at some point I shall have to upgrade to increase my capacity, I can certainly afford to do so, but will only do so if it becomes increasingly apparent that I'm going to need to do so...to many 'do so's' there
Keep asking questions on here and you will get the answers you need I am sure!
Ian