St Pierre des Corps is the TGV station for Tours, which is the transportation hub of the Loire Valley. There are usually trains from CDG to St Pierre des Corps, but these require reservations. Exactly which train you may want to reserve, after an all night transatlantic flight is always a question. Many people suggest booking nothing sooner than 3 hours after your scheduled CDG arrival to account for the many unknowns of flights, immigration, and luggage delays.
Another strategy is to go into Paris and take a TER train from Gare Austerlitz to the Loire Valley. These trains offer more options, cost less, and may get you to your destination a great deal faster.
It all depends upon your CDG arrival and your Loire Valley destination. I would check the train schedules:
https://www.sncf-connect.com/
https://www.thetrainline.com/en-us