I'm going to go ahead and do it manually, but before I do I would like to know if there is a better way
There is a way in edit mode you might prefer if you select the wall faces and press Ctrl F and choose Solidify. The thickness appears in the operator on the left. The new faces are automatically highlighted so at that point, you can add the new faces to a new group. Then if you make further modifications and want the thickness different you can select the faces from the group, delete and redo.