First of all, you must understand that SketchUp is not a CAD program. It uses polygonal models (from flat triangles), rather than solid models (described mathematically). Accordingly, after importing from any polygonal model into a dwg drawing, you will get Meshes and not 3D-Solids. They cannot be fully used in most AutoCAD commands. You will have to convert them to solids. This is far from easy and not always possible at all. I had to create a separate plugin (AVC MeshToSolid) because the commands available in AutoCAD did not cope with this task at all.