This article has a good history lesson...
https://rvshare.com/blog/rv-history/
...and an interesting quote I saw on another article when I googled "history of fifth wheels"...
We have to remember that the fifth wheel is the hitch on the vehicle, not the trailer attached to it.
I have a bunch of old Trailer Life magazines from the 70s and I think fifth wheel RVs as we know them today started appearing in the early to mid-70s. My Dad took us on our first summer trip with a bumper tow camper late June 1967, and we went on a 3 week trip every summer for the next 5 years. I rode shotgun in the front seat while my poor mother had to sit between my younger sisters in the backseat. I was very aware of trailer brands and makes since we had a Holiday Traveler. I counted Avions, Airstreams,Mobile Scouts and Holidays. I don't remember seeing any fifth wheel trailers back then - but my focus tended to be on the pool at the campground and the girls therein.
Also in my old 70s Trailer Life magazines are advertisements for RVs with "tilt-outs" - not slide-outs. They looked very cumbersome in the advertisement - which was trying to show it in the best spotlight. I seem to remember that slideouts did not become common until the early 90's. I can remember
lusting in my heart for those "huge" 33' long Alpenlites with a living room slide and how nice they were on the lot! Even Holiday Ramblers of the 80s did not have slideouts.