^^^ When slides first became popular Prevost did not yet offer them. As with everything market driven, if someone won't offer it someone else will. It is my understanding there were some shops that would add a slide to the standard non-slide Prevost chassis.
That would be the early days. I'm not sure how far you'd need to go back to find a non-OEM Prevost slide.
HERE is a discussion on adding a slide from 10 years ago.
HERE is another conversation from 2008 where the poster says:
"I recently contacted Robert Jones, Sales Manager for Prevost Car Co. regarding the slide outs on Prevosts. He informed me that 99% of their production is slide outs (not a real surprise). He also mentioned that Marathon, Featherlite, and American Carriage do not use Prevost factory slides (I assume they use Valid or HWH or some other manufacturer)".