I have been following and enjoying the discussion as to whether Peace is set in Kansas, Illinois or Ohio. After several months of my usual lurking, I figured it was time to settle the matter.
While reading Peace for the first time, I pieced together some of the clues (river, topography, William Quantrill) and concluded that Cassionsville might be in Ohio.
I asked Gene Wolfe about this and--in one of the rare times he definitively answered a question about a story of his--he confirmed that I was right. He also said that Cassionsville was set around and about the locale of his paternal grandmother's house in Logan Ohio. Wolfe spent some time there when he was a young boy.