You didn’t say exactly how you imported the SketchUp model into Unity, but it is clear that some step of the process involves a surface representation using only triangles (common to many representations). Your rectangular faces all have to be split into triangles in this representation because there is no such thing as a rectangle.
There are some additional problems as well:
In the style, I turned on edge endpoints and extensions before making the attached screenshot. You can see numerous vertices in the middle of or along the edge of (rather than at the corners of ) faces in your model. The export/import triangulation process is noticing these vertices and creating additional triangles as the only way it knows to deal with them. I circled some examples in red on the screenshot. You must either eliminate these vertices or force a particular triangulation by drawing diagonals of your own.
