Just to follow-up on this thread.
Not sure whether anyone else has been looking to replace this piece but here in Canada I had a heck of a time finding a replacement "shower swivel" without buying the entire shower unit. Once I started looking through U.S. websites things got a lot easier. I just received today a new swivel from Van Dykes Restorers in Mitchell, South Dakota. It is inexpensive and much superior to the swivel that came with our trailer - metal, not plastic.
If anyone is looking to replace theirs, and having trouble finding a supplier, you might check out:
Hand Shower Swivel Connector Specialty Bathroom Plumbing Parts Bathroom Kitchen and Bath from Van Dykes Restorers
Jay