It never occurred to me to try Tyvek. That's a great idea.
I bought a stock reflective windshield cover (shown below), plus I also have a set of three Heat Shield window covers for the inside of the cab - only when it's extremely cold do I put both inside and outside covers on (and they help tremendously).
Depending on vehicle parking orientation, the side of the van can present a worse heat gain issue than the windshield (especially ours, which has a hu-uge bank of black-masked windows running the entire length of each side, even where the wet bath is located). To my knowledge, nobody sells any kind of a side sun shield for a Class B. So I created a fabric shield out of a polyester liner overlain by a light-weight metallic that just happened to be on sale. Sometimes if one skulks around craft stores, there's good stuff to be found in discount bins. I think I paid about twenty bucks for all of this fabric, and a few more bucks for the neodymium "dot" magnets that I sewed into the hem of this to hold it in place. It works very well and folds down into something the size of a fat ham sandwich. I keep it in a mesh zipper bag so that it will not retain any moisture and become mildewed.
The only challenge with this project is having sufficient patience to work with neodymiums on a sewing machine, as they are going to stick tenaciously to as many parts of it as they can reach.
A blog post describing this project further is located
here.