Have you tried changing the texture in your version of Sketch up? It might be Ubuntu isn't associating the texture's file (if it was made from a picture file) correctly. I sometimes have trouble with "white" areas on models because the people who originally made the file used a picture file such as a gif or jpg, and then for some reason the association with the file broke, and Sketch up "lost" the texture info. I just then gave the model my own texture.
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