Best way to go from Charles de Gaulle to Gare Montparnasse

It is 10.30 by RER/metro

Yes but only 8 euros cheaper you can take the RER B suburban train to Gare du Nord and then on same ticket transfer to the metro line 4 Direction
Mairie de Montrouge.

You can look this up Paris rapid transit web site

https://www.ratp.fr/en

There's actually an easier way to go but it's counter-intuitive: take the RER B to Denfert Rochereau and change to the Metro 6 going toward Etoile. The 6 metro Is closer to the train station so you avoid a schlep.

On the 4 Metro you have long, underground 15-minute walks at both Chatelet and at Montparnasse. In both stations, you arrive at the opposite end of a giant station from where you need to go. There are moving sidewalks, but also Metro 4 has stairs going down under metro platforms and back up the other sides without escalators at Montparnasse. It's unbelievable that a metro station leading to a train station opened in the 1970s would lack escalators in so many places, but that's how it is. You see people struggling with luggage and baby strollers on these stairs all the time. It's a real haul. I learned the hard way.

Edit: If you do find yourself on the 4 metro, it’s easier to come up to street level and walk to the station than take the underground passageways.

Thanks - very useful!

Is it much of a walk within Charles DeGaulle from the arrivals to RER B?

What terminal at CDG, 1, 2A, 2B, 2C...? The RER is at T3 but you take the internal shuttle, Roissyval, to get there. It's easier than in the metro, flat or with elevators, and you can take a luggage cart most of the way to the RoissyVal shuttle train.

It's a pretty good hike through CDG. Follow ground transportation signs (I don't check bags). You will find a little driverless electric shuttle (Val) on a track that stops at the various terminals. Take that and get off at the RER stop (also for one of the parking lots). There are only a handful of stops, for the terminals and parking lots/RER.

As you come into RER there are a line of ticket machines and (as I recall) one ticket window opposite them. I always use the machines. I've found it's routine to have to try twice before it spits out a ticket. I've also noticed that sometimes it says the transaction didn't go through and still spits out a ticket. I believe when you buy your ticket, you can make it for all the way to Gare Montparnasse so one ticket gets you all the way there without having to stop and buy a metro ticket later. I think that also saves you a euro or so.

Without checked bags it takes me right at one hour before I'm on an RER B and leaving CDG for Paris. It's close to another hour to get downtown, and will take you at least another 20 to 30 minutes to navigate the metro to Gare Montparnasse.