Even your local WALMART may carry the RV 50A(M) to 30A(F) long dogbone adapter.
The adapter will only have one HOT leg of the 50A service connected to your 30A connection and should work like a champ...
I used to always use the 50A Service at the various campgrounds with my 30A Trailer as these receptacles don't get used much. Most everyone has the 30A trailers...
Just be sure your RV50A(M) to 30A(F) adapter looks like this...
I would be very cautious using someone elses wired up RV50A service connection. If it were me I would make measurements with my MULTIMETER to make sure it was wired correctly.
Measuring the 50A side at the 50A wired pedestal connection should look like this measurement...
The output of the RV 30A ADAPTER plug should look like this...
If you cannot verify these AC VOLTAGE readings as shown with your multimeter then I would most definitely NOT plug in the 30A trailer using an RV50A to 30A adapter into it...
You surely don't want a possible miss-wired RV pedestal connection to burn up your 30A Trailer appliances... That could add to as much as $3000 dollars to recoup your loses...
It wood really a bad situation to be in if something bad happens using the family provided miss-wired RV Pedestal connection...
Roy Ken