You could try emailing some of these
salvage yards for a replacement part, but the age of your RV might make it difficult.
Winnebago didn't have custom parts made for your RV, it is most likely a wiper mechanism used in other vehicles. If you pull the mechanism, or just take the arm off the shaft, perhaps an auto parts store can identify a matching replacement.
Also, while Winnebago doesn't show a parts list for a 1973, perhaps the 1975 or later catalog can help you identify the supplier.
Parts Catalog