If you have a progressive surge protector you would not have to worry about the 50 30 or 15 amp receptacle being wired correctly, the progressive simply would not allow the incorrectly wired receptacle to come on and damage your coach. If your coach does not have this type of device it would be a wise investment to install one before plugging to any receptacle at home or at a campground. In answer to your I believe most qualified electricians would know how to wire a 50 amp rv plug.
Good Luck Don