the rear beds may have started a little earlier. I have owned both a '72 & '73 ambassador (31') that both had the center bath- rear bedroom floor plan. My current '79 also is center bath, rear bed.
I didn't realize until you just mentioned it.....but I don't recall seeing any of the recent year models with rear baths.
No personal experience...........but I have heard both good & bad stories @ rear baths. Lots of stories @ rear "sag", I've never actually seen an example of it. Gotta give 'em one thing.....the ones I have seen sure have more space.
Skip & Betts