I do know a little about this business as I make & supply amongst others skirts to go around the bottom of catering vans, usually trailer types. My customers often make their own conversions, or buy from other former showmen who specialise in purpose built trailers or small truck conversions. The French have a fantastic range of small truck mobile shops, so you might nbe able to pick one up there, or take up Karl's suggestion. If you want an intro to the Showmen who make these things in Lutterworth, I can try to arrange it.
I know that in NYC it is all the rage currently to eat from mobile catering wagons offering really high quality and unusual take away food.
I was at the Goose Fair this year on business, as I was several in years past, and the variety of takeaways has increased imaginatively. I stick to mushy peas when there, (they were actually initially developed as the first takeaway but didn't catch on universally).