Very specific Paris train question: CDG to St. Germain des Pres

I hope this isn't too complicated or stupid a question, especially for those with lots of Metro/RER experience. However, I have never been in Paris before, but we are intrepid and determined to figure things out, but I would appreciate any help I can get. Before going tomorrow (flying from Copenhagen to CDG) I thought I'd go ahead and ask those who have experience with the trains and the Metro for advice. I need to get to the Rue de Duc St. Simon in St. Germain des Pres, which we have mapped as entirely walkable from Musée D'Orsay, where it appears a train (RER) leaves you. Is that true, or am I making it up? :-) The train station listed by my hotel as closest is Gare Montparnasse, yet that seems like quite a hike back up to the hotel, compared to how close the museum location is. If there is no train to Musée D'Orsay, can you please tell me where do we (or if we do) switch trains from CDG to get to St. Germain des Pres? I haven't yet tried asking my hotel all of this, but I can. I'd rather have experienced travelers' responses, though. Thanks very much!