The R/C Man wrote::o
Insulation.... It might not be as cold in CA but it gets pretty cold here in WA at night.
The insulation is more for moisture control. When you sleep, your body gives off moisture, and the interior starts to get mildew-infected, and smelly.
Ever smell a tent after a day or two of use?
Supposedly, this also causes damage to the TD over time.
As far as attaching the plywood directly to the trailer, as you can see, a lot of people do this. A frame is necesssary for a drop-hatch (which I don't have anyway), although I'm sure someone here has done it without one. (I don't care about those extra cubic inches of storage space.)
Finally, I like the idea of the frame because as said, it not only gives you something to attach the sides to (easier than drilling into steel!), but I can also envision doing something cool with baseboard trim around the entire perimeter. Plus, if you're running wiring, I imagine it's easier to attach your conduit to the frame than working around the steel of the trailer.
Also, as far as that Lowes AC goes, GET ONE WITH A REMOTE CONTROL! You're not going to want to keep getting up and moving around to adjust it. And why do you think you have to buy that now? Where are you going to mount it? I'm mounting mine in the lower galley (near my fuse box), to vent out the side, and I don't see any reason to need the measurements until I totally plan and START configuring the galley, which I see a lot of people do toward the very end.
Plan, measure, wire, start cutting.