I think I now understand what you meant by 'solid': the open spaces where you created the room to get the walls, that you would like to be filled in. The room you used is not a regular room, it is the 'inside' of a very thick wall.
Here's a trick to create your solid walls: You already have the walls, select the create room icon and click on a wall, not inside the walls. It will create a room with the outer circumference of the walls. Export/import, change the height to the height of the walls and you have a solid wall.
Remember that this is now a normal furniture object without the properties of a Wall object. You can place doors in or against it by moving them with the cursor keys but they will not create a hole.
...but because there are all different shapes it takes so long to do.
Keeps you from the street 
I've spend hundreds of hours on some projects, not to mention the many hours creating custom models. It's a hobby, the point is that it takes time! And this is a large project with very few repeating layouts, so hardly any use for copy/paste.----------------------------------------
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