Re: Can anyone tell me how to do this mysterious witchcraft of saving rooms?

@AbsentmindEric   Hi!  You do have to knock down some walls but then rebuild them using the custom room tool.  By doing it this way, these walls are not recognized as part of the room. Note that this means when you add wall coverings, you will not be able to use the shift key without adding them to the other "rooms".  So just add them by dragging per wall section.  I'm using the 19 Culpepper House Apartment as my example here since I think that's the one you show in your post.  Probably the best way is to start by deleting the walls with doors first. 

Then select the Custom Room Tool.

Redraw the now missing wall sections one at a time.  Click on one side, then click the other side, and then click back on the starting point; basically drawing two walls on top of each other.

Repeat those steps for each missing section.  Add doors and wall coverings back.

Now you have one big room to save, yet there are still individual rooms with doors.  Hope this helps!