Google Maps is tell me that. I should get on the RER B at Denfert-Rochereau, take it to Gare du Nord, then get off, get on the RER D to Stade de France- Saint Denis, then get BACK on the RER B to CDG. I feel like I should be able to just stay on the RER B to CDG and not get off at all no? Am I missing something?
RER B train is direct from Paris Denfert-Rochereau station to CDG Airport Terminal 1/3 & Terminal 2. No transfers or train changes are needed.
See the RER B line map at the bottom of this RER Paris to CDG article to see for yourself.
On occasion there will be rail works or disruptions that might necessitate making bizarre routes like the one you were suggested. But, otherwise, I'd go with the route with least transfers. It's the safest, least hassle option.