I agree that it's a simple one-train ride on RER Line B but the hotel's own website seems to suggest staying on for an extra stop to Luxembourg station:
http://hotel-luxembourg.com/pages/location.
For ChillDoubt's general information:
The RER train services operate like an express (limited stop) version of the Metro. If you've got the choice between taking an RER service or a Metro one, always take the RER as you'll get there far quicker and (in my own experience) they're usually far less crowded.
Both RER & Metro services (as well as bus services) are included with Paris Visite tickets, which give you unlimited travel within a specified area for one or more days. A 5-zone ticket (which includes CDG Airport) costs €23.50 for one day but you'd probably only need a 3-zone ticket for most travel within central Paris. Prices here:
http://www.ratp.fr/en/upload/docs/application/pdf/2014-04/partenaire_pv_2014.pdf