I use
Reflectix, it is an aluminum foil bubble wrap. It is easy to cut to size and lasts well. I used to use it on my pop up, saved the pieces when I sold that then cut them to fit my Airstreams. It comes in handy too if you want to completely block out light from any outside source like a street light.
It comes in a variety of sizes so choose the roll width that will work best for you. I also use some aluminum tape on the edges to give it a finished look.
It does a great job of keeping the interior cooler as well as reducing the sun fade, lightweight and doesn't take up much room for storage.
Aaron