goef the Z studs that you saw on the reefers still have a metal to stud contact so the metal skin will transfer heat or cold. The advantage of the Z studs vs tubing is that the foam has less gaps, the Z stud material probably being 1/8" or so thick. With 2" tubing you have a 2" gap between the sections of foam.

The Bigfoot class c motorhomes actually use tubing but they put foam inside the tubing. They are probably the best insulated motorhomes I have seen.

Patrick, Haulmark does not use FRP. I have looked at them twice. The FRP that Renegade uses is 5/8" thick with a plywood core. The FRP stands for Fiberglass Reinforced Plywood if I remember correctly. It was the Haulmark person who wondered why you would want to enclose the holding tanks and dump valves.

Haulmark uses a aluminum tubing frame with what their brochure calls "Gel-Coated Outer Shell w/ Luan Reinforcement". It is probably only 1/4" or so thick.

I hope to go to the Renegade factory this fall to see how they are built. One concern is the stainless steel corners (I think they are stainless?) that they use to join the side and end walls together. I wonder if there is any insulation in these corners?

Also, the term FRP is used for more than one type of product. The truck/semi trailer FRP has the plywood core and is 5/8" thick. Go to your local building supply and you will see FRP which is a 4' X 8' panel that is all plastic about 1/8" thick and has a pebble like finish on one side. I have seen it used on the inside of reefer trucks and was called Glassteck or something close to that. The truck body builder bought it in rolls that were at least 100 feet or more long and 8 feet tall. That way they could line the full length side or roof of a reefer semi in one piece. The size you find at the building supply is often used to line the walls of commercial bathrooms. I have seen in the bathroom of the grocery store and I believe that it was also in the bathrom at a Home Depot.