Go to a sales lot and look at the current Bambis with the bath in the corner. They are a neat little S&S unit. (that's 'sit' & shower)
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The roof arc is wider in the new Airstreams but one of the good guys in Airstream service at the factory told me he thought the unit would easily fit in the older configurations. How the H you'd get it thru the door is a horse of a different color. Maybe it is sectioned, but I dont know. He said the bath enclosure was first thing put in at construction.
You could find out who makes the bath enclosures for Airstream and see if they could/would help.
I have a full rear bath in a 31 ft, 74 Excella and consider the "tub" useless cept for kids. I would love to configure a corner shower stall, but probably never will.
BTW I gave thought to a metel stall and even to using a pie shaped shower pan with corrugated fibergalss wrap around walls, (like the stuff used on patio covers).Figured the pie would nestle into the corner. Wasn't clear how I'd seal the corrugated to the pan nor how I'd frame the whole thing. Twas just a daydream!
Best wishes and good luck.
Please keep this thread posted with your progress or lack thereof.