All are correct.
For a 30A cord, there are 3 wires - Hot, Neutral and Ground. It supplies 30 Amp at 120 V. THIS IS VERY DIFFERENT FROM A DRYER OR WELDER 30A PLUG!!!!! They deliver 240V!
For 50A service, there are 4 wires - Hot L1, Hot L2, neutral and Ground. Measure voltage between L1 and N or L2 and N, you get 120V Measure from L1 to L2 and you get 240V, just like a home electric stove.
The FIRST thing most RV electrical systems do is to separate L1 to N and L2 to N into two separate 120V supply sources. Thus, you get TWO 50A inputs, or 100A total power available.
The 30A cord only delivers one 30A input. This is why the 50A service gives so much more available power (3 times) than a 30A service.
