The Prevost XL frames are steel but not stainless steel and they are covered with alum and stainless steel body panels. The Prevost H coaches do use Stainless Steel with fiberglass and kevlar body panels.
The newer XLII uses a steel frame and stainless components plus stainless, no rivet, body panels to the bottom of the windows and alum the rest of the way up and forming the roof.
They have/had fiberglass front and end roof caps which I believe is still true on all models.