Back in 2013 I made some covers out of Reflectix for the Rolling Steel Tent's rear windows. They did a fine job of blocking light, creating privacy and providing a bit of thermal barrier.
I stowed the covers at the end of the bed, between my folding table and folding chair. Partly because it was a handy place, and partly to act as padding to keep the table and chair from rattling. It's essentially bubble wrap, after all. But over the years, the silver finish started to wear off, and instead of being opaque the covers became translucent. Then one of them blew away, never to be found again. (How can a shiny object about two feet square not be clearly visible out in nature?) I repurposed one of the side window covers, but it left a gap.
So I got some new Reflectix at Herb's Hardware in Quartzsite—one of the places you can buy it by the inch off of a big roll rather than buying a prepackaged roll of too much. I made new window covers and, presto, opacity returned.
With the window covers in place, the blackout curtain drawn between the cab and "living area," and the bedding pulled up to my chin so I can't see the status lights on the solar charge controller and voltage meter, it's really dark.
When you tour Timpanogos Cave in Utah, there's a moment they turn off the lights, plunging you into absolute darkness deep inside the mountain. It's a blackness darker than closing your eyes in a dark room, because your eyes are wide open, fully dilated, searching for something, anything. It's like this, only darker: