Let's see...
L1, L2, Neutral, and ground...
L1 to L2, 220VAC
L1 or L2 to Neutral, 110VAC
Ground is ground.
http://www.myrv.us/electric/Pg/50amp_Service.htm
Note the second sentence: "It is a misconception that this 50-amp RV service is something special. This service is a STANDARD 120/240 50-amp 3 pole with 4 prongs used for numerous applications."
30 AMP service, however, is NOT a three prong 220VAC service! It has L1, Neutral, and ground. It is 110VAC ONLY, and should never be connected to 220VAC!
http://www.myrv.us/electric/Pg/30amp_Service.htm