I have seen one. It was a fiberglass (like a boat hull construction), similar to a topper, but tal enough to stand in, and had a cab over, yet the sides were the same width as the truck. Gel coat exterior like a topper too. It was brand new and still smelled of the epoxy bc it was one day old, and the owners slept in it that night. They probably are dead by now from the carcenogins.
The interior looked like a topper with the rough sprayed-in fiberglass strands.
They intended to build it out also.
It attached same as a topper and maybe had the truck bed fully exposed, but not sure about that.
I cannot remember the brand, but just to tell you it does exist.
I saw it in NC, so wherever they got it was in NC or very close.
This was 5 yrs ago probably.
They were super excited about it and mpg was better than a TC.
Likely it could have been one of these as posted above
https://www.fleetwest.net/product/335/