Another one of those 50/50 pronunciation guesses. The proper French is as Scott Simon indicated with SHART-treh. However, we all know Illinoisans could never leave a good French name alone, and have shortened it. I think you'll find it both ways in that area. It's not unlike Lewistown in Fulton County. 15 years ago, we did a story asking residents how to pronounce the name of the town. The Guide says it is LOO-is-tuhn. The residents were evenly split - half said LOO-is-tuhn and the other half said LOO-is-town. I think in the case of the state park, you're right either way, Bill.
