Eliminating unwanted internal surfaces
I am sorry to tell you that SketchUp is not a good software for printable 3D designs due to its exporting feature not properly reducing complexity. It creates unnecessary faces and vertices inside of items on a regular basis, which in the slicer creates artifacts.
To make a SketchUp created file well printable, a huge amount of careful re-engineering in a graphics program like Blender is needed, manually removing the excess vertices surfaces.