The AC/Heater is most likely made by Evans Tempcon not by Ford. Should be a tag on the cover your looking for with their name on it. You might try some of the MH salvage yards, or call Evans.
Evans Tempcon, Inc
1999 35 ft. Dolphin 5350, F53, Banks System, 5 Stars Tune, Air Lift Air Bags, Koni Shocks, Blue OX TruCenter, TigerTrak track bars F&R, Roadmaster 1-3/4" rear auxiliary sway bar, 2004 F450 Lariat Pickup 6.0 Diesel Crew Cab DRW, 4X4, GVWR 15,000, Front GAWR 6,000, Rear GAWR 11,000, GCWR 26,000,1994 36ft Avion 5er, GVWR 13,700, 2,740 Pin Weight.