I have seen them at WALMART back in the camping section... Thats where we got our 30A to 15A long dogbone adapter from...
If you are going to build one up you will want to wire the 50AMP side like this adapter shows so that both 50AMPS zones in your trailer get 120VAC power...

Note the two HOTs are jumpered inside the adapter

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This adapter shown here is for using with your current 50A Connection cable at sites that only have 30A Pedestal Connections. Be sure to follow the same idea of jumpering the two hot leads if you are building up a 30A to 50A Twist lock adapter...
Both zones in the 50AMP RV will be fed by the same 30A source so you will have to watch what gets turned on other wise you will be tripping the source breaker.
Just my thoughts...
Roy Ken