Well, he was one. He definitely took some liberties with his story, of course (the protagonist becomes unstuck in time and meets extraterrestrials), but Vonnegut survived the firebombing of Dresden by hiding in an building named Slaughterhouse Five. He was captured by the Germans during the Battle of the Bulge, and he was part of the burial detail after the firestorm ended. He wouldn't have made up anything about POW treatment to make that experience sound more palatable. It pretty clearly haunted him and shaped his politics and writing throughout his life.

Without getting into major depth about the difference between American and Russian POWs, it's worth noting that the USSR did not sign or observe the Geneva Convention, so the Germans --even putting to one side the racism against Slavs-- did not feel compelled to do more for the Russians than the Russians were doing for captured Germans. Also, by 1945, most Germans would have known the war was ending. How badly were they going to treat captured Americans when so many of them who had the freedom to choose planned to surrender to American and/or British forces? As a final point, Vonnegut spoke German, so it's entirely possible he was just a little better off than some of his fellow POWs without knowing it.

For sources, I know Wikipedia is frowned upon. My edition of Slaughterhouse Five has a forward that talks about Vonnegut's capture, his experience in Dresden, and his thoughts about the war. For the Russians not signing the Geneva Convention, I can't remember a source off the top of my head, but I know it's true.