This is what you should have for the RV 30A Shore Power Cable to plug into...

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This will be wired just like the 120VAC 15A-20A Receptacles but will be fed with a 30A Circuit Breaker...
You MUST tell the Electrician this is being wired for a RV Trailer setup and not a 240VAC Circuit for a 240VAC Home Dryer appliance...
NOTE these two 3-socket receptacles that look very similar. The one on the LEFT is for the 120VAC RV setup and the one on the RIGHT is for the 240VAC Dryer setup.

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Forcing your RV Shore power cable into the receptacle on the right will destroy many 120VAC Appliances and system in your RV in a instance. This is a very good reason to use a licensed Electrician that has Insurance to cover these loses... you are talking about a few thousand dollars going up in BLUE SMOKE at the snap of a finger...
I am only passing this on for your education of the difference between the two very similar receptacles for two different system. I am NO EXPERT in any of this. Consult the Licensed Electrical Experts if you have questions...
Roy Ken