Sounds a lot like the old Travsak system. Basically a double sleeping bag with one side thicker than the other, you turn it over to change temperature. Sheets are cut to the same size and velcro to the bag. For some reason Travsak went out of business, despite having ridiculously high prices for what they were.
I like them because it is easy to make up, often a challenge in a camper. My wife doesn't like them because they don't overhang the mattress like a regular sheet/blanket, and she claims the cold air comes it (unless you zip it up, which is to much trouble for her).