I know a guy who has exactly the same problem as you.
Here is how he solved it:
1) He got a DI only system for his van with twin DI tanks.
(Unlike using RO, when you use DI the tank will fill as fast as the water comes out of the tap. Also there's no reject water. You simply attach the tap and fill. His system puts the water through the DI on its way out of the tank, so he literally just fills the tank with tap water, and it gets purified as he uses it. Very simple)
2) He went to a local petrol station and said to the guy, "please could I use your water every day to fill my tank. It is 400 litres, and I'll pay double what it costs you from the water company." the garage owner was happy to help, and he pays £2.50 every time he goes there. So he parks up, sets it filling, and has a cup of coffee and a read of the paper inside the garage while he's waiting. It takes about 20 minutes.
To me that seems like a great arrangement. Yes he has to pay about £10 a week to the garage, but its London, and look at how much you can earn in a week of window cleaning. Also, DI is more expensive than RO, but in his (and yours?) case it was a case of necessity. It's possible to buy pure water from here and there, but personally I'd rather be self-sufficient.
He got, on my advice (
), a Pure2o system (
www.pure2o.co.uk) which shuts off the water when the tank is full. He approached a petrol station, but you could equally go to a local cafe, hotel, or other business and they'll probably be very happy to help.
Good luck