If you are traveling light, the public transport options listed above are viable. Just be aware that the transfer from RER C to RER B at Notre Dame/St Michel is a couple levels down a stairway, since the RER C tracks are just below street level and the RER B is diving down to get under the Seine at that point. The No. 5 Metro is an elevated line at Gare d'Austerlitz with only stairs up to the platform. At least at Gare du Nord, you have elevators and escalators down to the RER B platforms.
I know this because I was staying in a hotel by Gare d'Austerlitz a few years back and was packing heavy and I spent a couple of hours scoping out my options to get to the airport the following day. I chose to use the No. 5 Metro, and next day as I rested on the landing half way up the platform stairway, a young lady offered to carry my suitcase the rest of the way to platform. Hilarious. "Non, merci!" I laughed and managed it on my own.