Paris: Elevator/Escalator access transiting from Gare du Nord -->the RER B to Luxembourg --> street

At Gare du Nord it is 3 levels down from the surface, where the main line trains are, to the RER. There are escalators for all 3 levels.
When you arrive, go to the front of the train, then left and you will see escalators down labelled with métro and RER signs. The ticket office is one level down.
This video shows the situation at Gare du Nord: https://www.youtube.com/watch?v=RSAHTKVBVwg
He is actually taking line D to Gare de Lyon, but it is the same for Line B, just a different escalator on the last level down to a different platform.

Sorry I don't know about Luxembourg station.

Métro stations in general have stairs more often than escalators. Most stations (the older lines) were built immediately below street level, and just have stairs down.

Yes, there is an escalator at Luxembourg RER station. It is on the North end of the platform.

Is the bus a possibility for you or is that too much like a taxi? I think you could stand on the bus even if it is not full. Possibly, it would be better for you than risking the escalator not being in service when you get to Luxembourg station. Bus 38 connects Gare du Nord and Luxembourg.

I was last in Paris in March 2017, and staying near the Luxembourg RER.
There was an escalator at the Luxembourg RER station working the day I arrived, but the year before it was out of service so I had to walk up to street level.
It's not far up to the street, as it's not a deep station, but perhaps you could pose this question again a few days before you go; as I know there are residents of Paris here on this forum, and they could perhaps kindly find out for you if the escalator is in service while you are there.
NOTE:There was no escalator going DOWN either time I was there, only upward.
I think there is only the one.
So you'd have to walk down stairs to get to the platform when you leave for the airport.
There were no elevators.
As well, a lot of hotels and apartments have stairs too; some have elevators to a certain part of the building, then a few stairs after that, so best to verify.
Seems like a taxi might be best for you, from the airport.
All Metro stations have a lot of stairs, except the one at Abbesses in Montmartre, in my experience.

Thank you. Good idea to ask a few days before my trip. Of course, the escalator could break down moments before I arrive. I won't have to worry about the steps going down to the RER B with luggage because my husband will be with me and he'll have my bag.

Thanks for the YouTube video, Chris F. Quick question, will I be able to buy the Découvert Navigo card when I arrive at Gare du Nord via Thalys and then go to the RER B using escalators or elevators thereby avoiding stairwells?

Every single time I have tried to walk up a staircase with a suitcase in Paris, some kind person has offered to help carry my bag.
No one has ever tried to run off with it! , but I guess you have to trust the kindness of strangers.

S Jackson, I actually do trust the kindness of strangers. Thanks. :-)

I do too, Continental!

Best not to depend on it, however.