Yeah, I remember the old Travel Queen slide-in and a lot of other ones as well.
A few years ago a friend, the guy who owns and operated the Avion website, told me about an Avion/Cayo slide-in truck camper for sale up in Indiana. It had been left in a barn when the original owner passed on. It was for sale cheap. It was in overall good shape, an 11 footer, but had some wood rot.
I pulled a flat bed trailer up to northern Indiana in the winter on snow and ice and bought it. The seller loaded it on my flat bed with a tractor front end loader IIRC.
I brought it home, looked it over, and decided I wasn't interested in the massive rebuild project to replace all the rotted wood. The Avion/Cayo was bad about rotting the wood in the side overhangs.
I sold it for five times what I paid for it and both the buyer and I were happy. I think the buyer rebuilt it and put it to use.