hmmm so many things to consider
how much space is needed then for a Ro system with a 200l tank, can an ro system make pure water on the go while im working
A 200 litre system needn't take up much room in a van. Here is an example of a 250 litre tank with its dimensions
http://www.andwat.co.uk/acatalog/ROTATIONALLY_MOULDED_BAFFLED_TANKS.htmlOf course you would also need a pump and, ideally, a leisure battery to power it.
It is also possible to get flat tanks where you can lay a board on top and place your other equipment onto it (don't know if you can get one in 200 litre size though). However, the problem could be with the RO. If you have reasonable water pressure you could make 200 litres overnight but you would need a place for the van to stand while connected up to the mains at night. Also, you would need some sort of drainage though that can easily be a small pipe down a nearby drainhole. The problem could be not with the production but having a place to leave the van etc where it won't be interfered with. There are much bigger (and more expensive) ROs that could make the water much more quickly - perhaps almost as quickly as you would use it.
A much better option (IMO) is to set up a static tank where water is produced 24/7 (or until the cutout switch stops it when full). You can then transfer this to the van via transfer pump and 1 and a quarter inch transfer hose. It's hard to know the best way to go without knowing your circumstances. For instance:- I live in a flat set back from the road so I had to make pure water elsewhere. I have a bit of storage space next to a stable (not mine) and just go there and top up the van tank when needed. I got a good deal on it and pay my rent with some free window cleaning for which I would charge something like £30 - £35 per visit. There is usually something like this about if you ask around. I imagine that you would have less difficulty than me in this regard as you are more community orientated than I am. If you think about it a bit, I'm sure you must know someone who could help you out for whom you could do something in return.
If not, you're welcome to some water to get you started. However, I do realise the importance of striving to be independent in business so I would understand any reluctance on your part.