Gary,
According to Prevost, the motorhome XLII version has a floor to ceiling height of 86 and the entertainer is 89 for the shell. I don't know about the ribs for the XLII, but will note that most entertainers have no ribs above the bay doors, but have a flat sheet of aluminum.
The heights for the XL chassis were for the shell as well. The actual finished interior height will vary based on the ceiling treatment used by the convertor.
Loc