Mike Watters rebuilt and modified one from the ground up within the past couple years.
I bet he is most intimate with knowledge on these.
I've seen very little info, or other examples for that matter... Mine was titled as a '72. The one on eBay now I think is listed as a '75. That's as far of a range as I know of. Don't know where made.
They have a somewhat freaky double wall construction. It's actually a kind of laminate of 1/4" sheet foam with fiberglass on both sides. Looks like the foam was laid into the molds as part of the molding process.
Although I have photos - I've never actually seen one with the original floorplan (big dinette/bed in back, front kitchen, corner bathroom. Mine had been redone once already when I bought it. (photos in the link in the signature).
If I were going to do it all again - I'd do something more along the lines of the original floorplan. Although the front dinette was very appealing - the slope on the front of the playpac made it a problem. Well, for my 6' self anyway - the kids didn't have a problem...
One nice thing... since they're a little wider than a normal egg - I was able to fit a normal residential queen mattress into it (the entire floor plan was designed around it in fact!). HEAVY but very very comfortable!
I assume your's is more or less stock???