They are probably a part that Fleetwood used in many coaches over many years, so chances of finding them in an RV salvage yard are good, or maybe even new ones are still available from Fleetwood. Further, they will be from an regular HVAC industry component source and not unique to Fleetwood, so searching around online should find a replacement as well.
Proline is one manufacturer of those vents, and Summitt Racing carries several models.
https://www.summitracing.com/search/part-type/interior-vents
Or search around on this site:
https://www.1aauto.com/dash-vents/c/269