Transport from CDG to Left Bank

I believe there is an elevator. Side of the building at Rue Xavier Privas and Quai St Michel.
https://www.google.com/maps/@48.8532674,2.3458778,3a,75y,157.51h,92.12t/data=!3m7!1e1!3m5!1soRJbkRjJSAfuH5zqwCCKzg!2e0!6shttps:%2F%2Fstreetviewpixels-pa.googleapis.com%2Fv1%2Fthumbnail%3Fcb_client%3Dmaps_sv.tactile%26w%3D900%26h%3D600%26pitch%3D-2.11928969851553%26panoid%3DoRJbkRjJSAfuH5zqwCCKzg%26yaw%3D157.50976909362387!7i16384!8i8192?entry=ttu&g_ep=EgoyMDI1MDIyNi4wIKXMDSoASAFQAw%3D%3D

From RATP site:
https://www.ratp.fr/en/services/etat-equipement/rer/b/ITIV_IDFM_C01743_A_-139787549/ART_IDFM_44877

Transalation: Location: In the middle of the RER B platforms, to access exit 6 (r. Xavier Privas), next to a ticket machine, in a recess / Outside, at the corner of Quai Saint-Michel and Rue Xavier Privas

It says "out of service" but it is as of 1:21 am. You can check the site when you get there,

I'm not sure of the escalator situation.

But, there is an elevator on the platform level that will bring you up to rue Xavier Privas and Quai Saint Michel. Follow the signs for Sortie 6.

I distinctly remember coming off the escalator and sort of mesmerized that it had dropped us off in front of Notre Dame, but it was so many years ago, I have no idea if things have changed. OK, we're in Paris!

When we were there last spring I have no recollection of seeing any of that, but I wasn't looking for it.

The challenge is that there are multiple exits from some stations, including St. Michel. We stayed near there and it seemed that every time we ended up taking a different exit. Most are multiple levels of stairs. There is an elevator at the corner of Quai St. Michel and Rue Xavier Privas. If you use google maps street view, you will see a blue RER sign. The elevator is right around the corner from the Amorino shop. It appears that there is an escalator that brings you up in front of Notre Dame. Whether they will be working when you arrive is another question. Be prepared to carry your bags up many steps, in case they are not working.

You can use google earth to walk around the area and see the various RER entrances.

Now that it is 7:30 am in Paris, the RATP site says the elevator is in service.

All RER stations in Paris have elevators. Métro 14 as well.

There is an escalator on the southbound (arrival from CDG) platform of the RER B at St Michel. This escalator, on the departure end, leads to a moving sidewalk, ending at the Blvd St Germain and the Cluny métro station on line 10.