Charles de Gaulle Airport to Chartres on April 13th 2017

5 hours? Good Lord, www.bahn.com says 2 1/2 hours tops. Now it could be more than that because the trains from Gare Montparnasse to Chartres only go about every hour, so you could conceivably arrive at Montparnasse just as the train is leaving and you would then have to wait another hour for the next train.
Bahn suggests taking the RER B to St Michel/Notre Dame, then changing to the No 4 Metro to Gare Montparnasse where you pick up the train to Chartres. That will leave a rather long walk (with a moving sidewalk for part of the way) to get to the train platforms. I'd probably opt to remain on the RER B to Denfert-Rochereau then taking the No 6 Metro to Gare Montparnasse at a station closer to the mainline train platforms.
Another option, a little more expensive and a tad slower, would be taking the bus to Gare Montparnasse, then train from there. You can store luggage in the bus hold without having to traipse it through the Paris underground stations with stairs.
http://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/public-transport/le-bus-direct-line-4

What are you looking at that is 5 hours? Yikes. You go into Paris Gare Montparnasse which takes roughly an hour (you can take a bus or take the RER/metro) and then take a train to Chartres which takes about an hour and a half at the most.

I suspect that the OP is looking at Rail Europe (which often only checks a few long-distance trains), and that it's suggesting using a TGV from Charles de Gaulle to somewhere out towards Brittany and then taking a train back east to Chartres.

The best way to go is to take the bus, Les Cars Air France, from CDG to Montparnasse. Then you'll take the TER to Chartres. The terminus will either be Chartres, Nogent-le-Rotrou, or Le Mans.

Now renamed Le Bus Direct. Line 4 is the one you want.

One hour exactly is the best transit time (by TER) from Paris Montparnasse to Chartres.
I would think about 4 hours from touchdown at CDG til arrival at Chartres.

Check how many stops with each departure time. Some trains stop only a couple times while other stop many more. This would add to your travel time.

Some of the trains make 4 intermediate stops and take 1 hour and a couple minutes, and some do it with 8 intermediate stops and take an hour and 16 minutes. Not a big deal.

I am SO appreciative for all this help. I am amazed at the generousity. With great gratitude, Lilli*