I don't have the timetable, but I can tell you that catching the RER from any arrondissement will be very easy. Take an metro train to any connection along the red line (la defense). Once at a red line station, take the RER to the last stop on the 'Marne La Vallée - Chessy' bound trains. What out, though, as there are two branches on the RER going east. If you find yourself on the wrong train, get off at the next stop and wait for the right one. Trains run every 10 to 20 minutes, and it takes 35-45 minutes to get to
Disneyland, depending upon where in the city you begin. The prospect of your train being full is slight. We visited on the Monday after Easter, which, to our dismay, turned out to be a French holiday. Still, lines were short and the trains were full of empty seats.
The trains begin running very early -- you will have no trouble getting to the park before it opens, and they run well into the evening, long after the park closes, especially this time of year, when the park closes fairly early (8 p.m.) for non-hotel guests.
The train drops you off maybe 100 meters from the ticket booth, and its proximity will be a welcome feature when your tired legs leave the park at closing time!
Hold onto your train tickets, too. You will need them to get out of the train station when you arrive, both at Disney and at your point of origin (or, more specifically, the point where you leave the RER line), otherwise, you might be banished to live in the Paris underground for all of eternity.
For more information, this website has a nice page on the RER with links to Disney and other useful sites.
There is a nice page at a website called parisdigest, but DIS won't let me post urls yet, since I'm so new and all, but you can find it be googling "rer paris disneyland"