Corner or midships twins? I was able to get twin mattresses to fit my '74 Argosy at Camping World. They're quite similar to the now standard issue college long twin mattress (J.C. Penney's being a good source for sheets).
Though expensive, Bearbeds are an alternative to the mothership. Makes the mothership look reasonable, eh?
If your thinking of buying the twins with the rounded top edge from the factory, I can tell you that the mattresses that came with my 2007 Classic are junk. On a scale of 1 to 10 they're at most a 2. Better than sleeping on the plywood, but just.
Wouldn't a good foam setup be easier to put together? Dunno about cheaper, but the foam at least you can cut yourself with one of those electric carving knives. One of the long renovation threads had a lot of info on them...
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