It's a variant of those windshield things that they sell at truck stops to keep the sun from baking the car. In fact, going to a truck stop and buying a dozen would be the easiest way to follow this advice. You would then put them behind every window, shiny side out, and maybe gain 30 extra minutes of sleep in the morning.
The problem is that cars (and vans) are not well insulated to begin with, and to truly get something that would not warm up in the sun, you'd have to put in an inch of fiberglass all the way around. What works best with cars for sleeping would be to put a shade over yours, with at least a foot between the roof and the shade, for air to circulate. Then you could open the back and all the windows and be pretty comfortable.
Or you can do your night sleeping in the van and your day sleeping in a separate shade structure. I don't think mylar's going to buy you much time, but I'm not one of the expert shade people. One of them will be along in a few minutes to give you much better advice.
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"So many people have so little clue and there's only a finite amount of patience in the world!" --International Incident.