We built our own trailer for bbq competitions at a cost of $7,500. It's a 16 ft Doolittle that's perfect for us. It's still a work in progress, but we've used for two seasons now.
Here's a link to a company that specializes in building concession trailers in all shapes, sizes, designs and layouts. They've got a porch concession trailer that would allow for mounting a cooker on the back.
Olympia Blends Products and Services LLC
Based on the information on their web site, the trailer comes with linoleum, vinyl wall covering, grey water tank w/vent, fresh water tank, hot water heater, 100 amp wiring and GFI plug, etc.
It looks pretty nice. You might build it for less, but the headaches you'll run into might be worth the money!