Traxman,
Most folks do not realize the differences between the entertainer chassis and the Prevost motorhome conversion chassis. Apart from the obvious wheelbase length and the amount of stainless steel on the sides, the windows are different.
Motorhomes have awning type windows and entertainer coaches have sliding windows. It is doubtful if any motorhome owner can steer you to the source for the sliding window gasket, but the folks that might be able to help would be the operators of entertainer coaches.
The following link is to the entertainer motorcoach operators who are part of the American Bus Association, and I suspect someone on the list has the answer you seek.
http://www.buses.org/EMC/EMC-Members