@Gretchen,
I was going to reply to your questions by PM, but decided to reply here in case the answers might be useful to others.....
but do I need to buy a ticket before I get to the airport for the RER B train (meaning online before I leave the US)?
No, you can buy your ticket for the RER when you arrive in Paris. There's no benefit to buying online, and I'm not sure if that's possible. The ticket kiosks ONLY accept C&P credit cards or Euro coins, so without a C&P card you'll have to stand in the queue at the ticket office to buy tickets. You may find it helpful to have a look at the excellent Paris By Train website as they have lots of information on how to travel from CDG (including photos of the route to follow in the airport). Be sure to watch your luggage when on the RER as there could be scammers on board.
I was planning on just waiting for a train upon arrival. Can I still do that now, even if I need to wait much longer, due to the strike
As long as the strike isn't an issue, you likely won't have long to wait as there are frequent RER departures. Just buy a ticket when you arrive and then board the next train that's leaving.
Will these trains be so overcrowded that, I should have a ticket ahead of time?
The trains may or may not be crowded, depending on how many flights have arrived at about that time. There are no seat reservations for the RER, so you may end up standing for part of the trip, but in all likelihood you won't have any problem finding a seat. A few important notes from the Paris By Train website....
"Remember to keep your train ticket with you at all times throughout the journey. You may be asked by a ticket controller to show a valid fare or otherwise pay a penalty fare of 35€. The tickets are also required to exit the arrival station in Paris as there will be turnstiles protecting the exits, just as they protect entrances. Finally, your RER train ticket is also good for connections onto the Paris Metro and other RER train lines once you’ve arrived in Paris; There’s no need to purchase a separate Paris Metro/RER Ticket T+ for making connections within the city centre onto other Metro or RER trains (but not buses nor trams, which require a new ticket)."
I will need to take a train from Gare Montparnesse to Chartres on Sunday the 29th to start my RS Tour. Is it likely these trains will be affected as well?
Based on the information I'm reading here from Kim and others, it appears that the rail strike is winding down. No one can say for sure whether there will be disruptions on that route on 29 June, but there will probably still be some trains running even if there is strike action. Hopefully you won't have any problems.
Enjoy your tour!