Reflectix is not much good as far as insulation goes. It makes a convenient windshield cover.
Sheet foam insulation may be an option. It's easy to work with and readily available.
The highest R value per inch of insulation board would be Polyisocyanurate with an R value of 6.5-7per inch. I don't care much for working with it as it crumbles an makes a mess.
Extruded Polystyrene (XPS) is my preference but only R-5 per inch.
By far the best solution is:
https://www.amazon.com/Reliable-Outdoor-Gear-Professional-Compass/dp/B0759LHB92/ref=sr_1_4?keywords=Compass&qid=1560192380&s=gateway&sr=8-4
If I get unbearably warm, I point the little pointer at "N" and step on the long skinny pedal....
2004.5 Dodge 3500 5.9 Cummins, NV5400, 5" turbo back stainless exhaust, Edger programmer & 22.5 Alcoa's
2002 Forest River 36 5th Wheel (staying home)
1992 Jayco 29 5th Wheel (Mexico veteran & headed back)
2002 "faux" Wanderlodge 40' My new toy....