You might try to find an ambulance body off a dually van/truck. They vary in size but average ones are 12' long, SUPER strong alloy aluminum extrusion welded joints and well insulated. I found one lightly rolled, awsome find for $400, I talked to a few guys who scrap the older aluminum ones and they said it is hard to crush them with a trackhoe. It is skinned in 1/8" aluminum. The one I have is a Lifeline type I or II (dont remember) but I would bet it would cost 40k just for the rear body new. As a bonus there was a bunch of electricals left in it. You could then use tires and axles of your chosing, I am using 17.5" singles good for 6000lbs each on a custom frame.


http://www.lifelineambulance.com/products.html
Ironwood