In a thread a year or two ago, a military contractor lived in his Airstream at a remote site at a military base in the desert southwest. He made cutouts from "sheathing" used in home construction, to fit in all of his windows and vents. Not sure, but he may have also covered some of the walls. Perhaps, he will see this thread and provide more details on how well this worked.
Sheathing is a relatively inexpensive, styrofoam sheet with a white plastic sheet laminated to one side and aluminum foil on the other. It comes in 4x8 foot sheets that (I think) are either 3/4- or 1-inch thick. They can be easily cut with a serrated knife or electric carving knife, although it is kind of messy; because the little styrofoam chips go all over the place, and they are charged with static electricity and cling to everything.
This extra insulation helps quite a bit, because a lot of the summer heat gain is through the windows. Unfortunately, this will also turn your Airstream into a dark cave; so you will have to decide if being cool is worth sitting in the dark all day.