Hey MGM_Home, I see that you have not found (or at least posted) a solution to your problem. I had the same issue and after watching some videos and researching other similar questions, I have a working solution for me. It might work for you too. Essentially, I drew two additional external walls into the room, long enough to intercept with the dormer's ceiling planes, then edited the wall to "Roof Cuts Wall at Bottom". There is still a small piece of the dormer's ceiling plane that I cannot figure-out how to eliminate, but at least it solves the weird drywall issues without the extra walls and closes the "gap". My next step is to see if I can hide those extra walls in the plan view. Hope that helps.
UPDATE: After posting this, I went back into the model to see if I could hide the extra walls (making them invisible didn't produce the desired results). Anyway, I read about hiding attic walls, so I tried redrawing the dormer roofs without raising the ceiling in the room, which created attic walls to close the "gap". However, that created walls above the roof as well as below. I was able to use "Roof Cuts Wall at Bottom" to get rid of the wall in the room, but could not delete the wall created above the roof, without it re-creating the wall below the roof. Anyway, still not the best solution, but going with my original method until I can figure-out how to create a better solution.