CDG airport to Montparnasse train station?

Why yes there is and it is quite easy and almost stress free!

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I take it almost everytime I arrive in Paris. Just make sure you leave enough time to get from the airport to the gare.

Off the top of my head, the closest RER stop is Dunfert Rochereau which is not far (to walk with luggage, yes, but you could take the bus or metro). To me that would be a pain in the neck because you'd have to walk all over with your luggage and any metro with luggage equals stress.

Which Paris airport? RER trains cover the Paris region only. La Rochelle is a common name including streets around Paris.
If you are landing at Charles de Gaulle airport and going to the town north of Bordeaux, the national rail company SNCF has TGV trains, the fast ones, departing from the airport SNCF station every couple of hours. There is one change of train and the journey takes about 4hr20, according to the SNCF website.
The trip on a train from Paris itself takes one hour less and does not require a change of trains. However, you have to make your way from the airport to the Montparnasse station which will probably require changing from commuter train to the Metro, taking at least an hour and adding to the cost. A bus from Air France Cars to Montparnasse is more comfortable but will cost more than the RER/Metro, and can get caught in traffic. At Charles de Gaulle the RER and SNCF stations are adjacent to each other in part of Terminal 2.
You will be a more confident and comfortable traveller if you research these itineraries yourself.
Here are some tools, not the only ones:

http://en.voyages-sncf.com/en/ The French inter-city rail system.

www.seat61.com The encyclopaedia of rail travel.

http://www.aeroportsdeparis.fr/ADP/en-GB/Passagers/Home/
The terminal maps are helpful.

http://www.ratp.fr/en/ Interactive guide to Paris subways, buses, RER and streetcars.

www.parisbytrain.com Guided tours of some Paris stations with photos.

Hints: SNCF fares on main lines are cheaper when bought well in advance and will include seat reservations when required. If you live in the US/Canada choose Antarctica as SNCF ticket pick-up. You will be able to either print out your tickets at home or retrieve them from any SNCF station or boutique. Eurail can cost you more more and give fewer choices.

In the event that you decide to head to Gare Montparnasse from CDG, you can get information on the Air France bus at www.lescarsairfrance.com or you can take the metro/RER combination of RER B to St Michel and change for Metro Line 4 getting off at Montparnasse-Bienvenue and following the "Grandes Lignes" signs to the SNCF trains. I'd probably take the Air France bus -- less walking with luggage -- or the train to La Rochelle from CDG.

I agree -- take the Air France Bus. It will take you right to the station. Check the info about it on the link posted above. Be sure to take the correct line to get to Montparnasse. These are very comfortable buses, where you can sit and relax. I always use them.
No need to worry about standing on a crowded train with luggage and making train connections in a metro station -- which is not very much fun.

Yes, take the Air France bus. After you get your luggage, there is a nice friendly lady at the tourist desk right at baggage claim who can direct you to where to catch the bus. Just walk to the right gate number. She will give you a map. You can buy your ticket in advance on-line if you don't want to hassle with the automated machine at the airport. They will load your luggage onto the bus' underneath compartments.
Gare Montparnasse is the last stop. It will stop right in front of the station. Just turn around and go up the stairs.
It is a more relaxing ride than hassling with the stairs on the RER. You get to see other parts of Paris, so think of it as an introductory tour.