Roissy train station at CDG

If you mean the tgv station, it's in Terminal 2, about at the middle.

There are two rer stations, the one at the T2 tgv station, and another one at T3/Roissypole.

There are shuttles, stairs, ramps, escalators, and elevators all over the place.

You may find it helpful to have a look at the excellent Paris By Train website. It provides lots of good information on the rail stations at CDG.

Thanks to both of you. A travel agent told me I should go into Paris to catch the train to Avignon, but I realize I can easily do it from the CDG TVS station. The French train site is helpful.
Marilyn S

Trains from Charles de Gaulle TGV station depart direct to Avignon about 5 times per day - less often but more convenient than from central Paris. If booking a non-refundable train ticket in advance, allow plenty of time to connect at CDG. Three hours sounds safe, but a flight delay could also wipe out that time.

Rick’s France rail travel page has the link and tips for buying “Prems” rate TGV tickets through SNCF (cheap, reserved, nonrefundable, available three or four months ahead of your travel date) to print at home and pay with PayPal. It does not tend to work with US credit cards, unless you have an extra level of security set up.