When reading this question I just happened to recall a Soviet writer, who was Jewish, who I think did see the concentration camps and write about them but wasn't a victim of them.
But, this person didn't write in favor of communism despite being a writer in the Soviet Union. Their books were censored.
I found his book "Everything Flows" to be extremely interesting. It is a fictional account of a Soviet gulag survivor returning to his home, and reflecting on the people he meets and his life.
