Meshlab

MeshLab can be useful. This tool is your 3D-geometry studio on a budget. With MeshLab, you can do some of the housekeeping tasks you need to do to take your SketchUp asset model and turn it into a game-ready article. It's completely free, but it's an "in development" project, so you might have to wrestle with it a little at times.

You can download Meshlab at http://meshlab.sourceforge.net/, and installation is straightforward. I recommend downloading the "stable" version. Go to the side menu and select the Windows, Mac, or Linux version from there.

When you have installed the software, allow it to run, and you will get the following screen: