Although a 50 amp service for a recreational vehicle, or RV, is connected to four wires and uses a four-prong plug, it is still a three pole service with only hot, neutral and ground connections. It is different from a conventional 120 volt service in that there are two 120 volt hot feeds, or legs, each at 50 amps. A 50 amp service is not a 30 amp service and a 20 amp service joined together. Even the largest RVs, when plugged into 50 amp service, still almost universally use this double-hot-pole, or double-bus, installation to draw only 120 volts to two separate 120-volt circuits simultaneously, thereby accessing a possible 12,000 watts of power.
The outlet is called an NEMA 14-50R. Buy a 50 amp outlet which is pre-installed into product-specific, self-contained, weather-proof box. Buy ROMEX® #6 wire, meaning each hot wire and the neutral wire are six-gauge.