I have seen several when I worked in Texas during the summers in college, not sure what the added cost were but man were they sure nice to have at summer shows down there like the belt buckle! One of the familes the guy I worked for down there had one and it was an awesome set up had a thermo king on it!
I know of one where they simply framed and insulated a "box" for the front trailer compartment, left enough room to have a small window unit sticking out of one side, and use a designated generator running in truck to power it.