Was McMurphy a WW2 vet?

No. MacMurphy was a Korean War vet, who was dishonorably discharged for insubordination. In fact, Nurse Ratched runs down his history when she reads his chart:

McMurphy is interred at the hospital for "diagnosis and possible treatment," reads Nurse Ratched, who continues: "Thirty-five years old. Never married. Distinguished Service Cross in Korea, for leading an escape from a Communist prison camp. A dishonorable discharge, afterward, for insubordination. Followed by a history of street brawls and barroom fights and a series of arrests for Drunkenness, Assault and Battery, Disturbing the Peace, repeated gambling, and one arrest — for Rape." (taken from the Cliff's Notes page)

While it's true that the Korean War succeeded WWII by 5 years and RPM would have been old enough to have fought in it, there is no indication anywhere that he was involved in anything other than Korea.