We'll be traveling from CDG to an apartment on St. Louis. I'd hoped to take the RER to chatelet and change to a line 7 metro to Pont Marie. Now I'm thinking Chatelet is quite a zoo, is there a map of the RER/Metro station? Or are they two separate stations? Also, when I try to use the RATP interactive site, it only puts me on the #1 line to Hotel de Ville??

There are two stations: Chatelet, and Chatelet-les Halles.
RER-B passes through Chatelet-les Halles, but not Chatelet (the map shows a long walking correspondence).
If you use the RER station (Ch.de Gaulle 2) and the Metro station (Pont Marie), then RATP will give you what you want.
RER-B to Chatelet-les Halles, walk thru correspondence (7 minutes) to Chatelet, take Line 7 to Pont Marie.

The walk thru correspondence Travelnut spoke of is well marked, just read & follow the signs, you will have no problems. On our very first trip to Paris took the RER from CDG to the Chatelet-les Halles stop and walked through to Chatelet (our hotel was the Hotel Victoria Chatelet) it was very easy - we had 1 roller and 1 carry on. If you are traveling with a lot of luggage you may want to re-think, otherwise it's fine.

Thank you so much. With 3 of us (only I speak french) we should be able to find le correspondance. My brother mentioned that, if it's rush hour, Chatelet is no place to be. I assume we need an RER ticket plus a Metro ticket (orange or otherwise)

The RER ticket from CDG covers your ongoing metro ride as long as you are corresponding within the system, not leaving the system (going thru the exit turnstile).

I must warn you that the walk from Châtelet-Les Halles RER to the line 7 platforms is the absolutely longest one in Châtelet. However, you will be taking 2 moving walkways on your way to line 7, so you don't really have to actually walk a tremendous distance -- just don't be surprised that you keep going and going and going and never seem to arrive.

Maybe we'll just take the RER to St. Michel and walk from there to our place in St. Louis. Looks like it's under half mile either way

We did that same trip and got off at Sully Morland. There is no problem changing at Chatelet. As one other poster stated, just follow the signs and you will get there.