Thanks to You Guys, Mystery Solved!
This is part of the Swan Flushing System SFS 200 City-White, for my black Water Flush System. This is the Vaccume breaker between the city water and the black water for the flush system.
I put Your comments of vaccume breaker and rv waste system and googled that, wow, lots of information about this exact system. I confirmed that this is the same part by looking at the ring around the city water inlet for the flush system and sure enough it says SWAN SAN-T-FLUSH.
Ordered a replacement on eBay for $37.00 (incl shipping).
I would never have connected this to the black water flush system. It was located just under the small bathroom sink, across the hall from the black tank/toilet.
Thank You all for your help.