Hi Lucia,
Actually, the RER B stops very close to the Marriott Rive Gauche:
see here on Google Maps.
When taking the
RER B from CDG Airport, stay on until station Denfert-Rochereau, then you can either walk along boulevard Saint-Jacques until the hotel (
about 6 minutes walk) or take Metro Line 6 direction Nation for one stop, getting off at Saint-Jacques station and you'll be steps away from the Marriott.
When you exit either the RER station or Saint-Jacques Metro station, you might have a couple easy landmarks to help point you in the right direction. Montparnasse Tower (50 odd story skyscraper) is the highest structure in Paris besides the Eiffel Tower. You want to head away from the tower along Saint-Jacques. If you can see the Marriott from the station, even easier.
La Vallée Village is right along the RER A train line that goes out to Disney. See my article on taking the
RER A from Paris to Euro Disney and simply get off the train at Val d'Europe station (one before Marne La Vallée, the Disney station and end of the line).
I'm not sure if you've got incredible luck or you planned it out all perfectly but your itinerary jives perfectly with the RER train nework. Nice work!
Have fun!
Ben