Won't Rev Group mold a new one for you? They take months and are expensive, but supposedly their Goldshield subsidiary has the dies and can mold any Fleetwood/AC body part needed. I got a new lower front panel from them for my '04, but that was several years ago now.
A n RV salvage yard would be quicker and at least a little cheaper, though. Colaw, Visione and Arizona RV are the big ones, but there are dozens around.
Here's one list: http://www.marxrv.com/rvsalvage.htm