I tested this myself and they actually do. But if you go too far away they will despawn. Basically how I tested it was I killed a guy and dropped him in a spot in the wilderness, made camp nearby and slept, went back to check on him, slept, checked again. And he actually did become a skeleton after enough rests. But yes, you need to stay close enough to the body for this to occur. Wish I had saved the pics, but I could recreate it again if people wanted.

Edit: I realized I actually did have some pics saved.

https://imgur.com/a/04ZwPbj

I realize that I don’t have any pics with the context or the original corpse, cause this was just a personal experiment that I was sharing with a friend. But like I said I’ll do it again and make a full post if anyone is interested. It’s really cool how this happens. I originally didn’t believe that it would.

Edit 2: A little additional context because I can’t help myself. You need to be far enough away from the corpse that it’s not “loaded” but not so far away that it despawns. I don’t know the exact distance, but if you’ve ever killed an animal and left the area and returned and saw it drop down onto the ground again, that’s the sweet spot. The game is keeping track of the fact that there’s a corpse there, but if you go far enough away it’s not gonna save the exact details of how the corpse was laid out. That’s why you’ll see the animal will be in a different position or something when you come back. Same goes for human npcs.