I had briefly considered this myself. I decided against it after I thought about the time & hassles of ripping stuff out, re-paneling, etc. I viewed many that are for sale on craigslist, ebay, etc.
Another consideration - A nearby city allows carts / trailers downtown, but they cannot be homemade (or converted travel trailers).
When looking at existing "campers converted to concession trailer" for sale, I asked myself, "would I buy food from THAT?" My personal answer was probably not, as most of them looked really tacky. Like many people, I do not have the $$$ to buy a professionally built trailer, but I want something that looks decent that will draw customers in, not drive them away.
I bought the E-Z built hot dog cart plans & going to start with a small cart, then hopefully grow into a trailer that I will likely have built by a company.
E-Z built cart plans: http://hotdogprofits.com/
Company I may likely choose to build trailer: http://www.allacart.com/
I've not seen any plans for conversions. Just look at photos of existing stuff. The main thing is deciding where equipment would be set to make food prep / serving most efficient.
<message edited by hermit on Thu, 11/5/09 10:41 AM>