Take a taxi. For a couple of reasons. First, that area isn't well-served by the métro, especially if traveling with luggage. You'll have to change, and with luggage, schlepping it up and down all the stairs in a métro station isn't fun. Also, you don't say how many are in your party, but a taxi is a flat rate of €65 to the Left Bank. No tip is required. A single ticket on the RER is €13, so if there are more than two of you, it's a no-brainer when you're that tired.
From baggage claim, follow the signs to the official taxi rank, ignoring anyone who asks if you want a ride along the way. They're rogue drivers who will really take you for a ride.