This is a 50AMP setup that might give you an idea what to look for...
Google image
When measuring the 50AMP Receptacle it should look like this ...
50AMP
Google Image
30/20/15AMP
Google Image
Most RV's use the 50AMP connections as two zones of 120VAC ONLY.
Only very few high-end RV units actually use the 240VAC connections usually for the Dryer connections...
This is the only way a RV Unit frame gets grounded going through the shore power connection. I would definitely not ever use a connection that did not have the earth ground connection...
Roy Ken
