Prior to the 70's, most campers only required a standard 15 amp household plug. They started installing Air Conditioning units in the 70's which required the higher 30 amp plug.
This became the standard 30 amp RV connector. In the 90's the bigger rigs used more AC's so they started using 50 amp service. Basically, all sites have 30 amp service however most also have 50 amp service also in the same electrical box. Not unusual to also see 15 amp, 30 amp and 50 amp service in the electrical panel at an RV park. They make adaptors to to convert between each of style of conector.