There are a few faces in the sketchup files that need reversing - they're internal rather than external. The default colour scheme makes it a little difficult to pick them out, but you can change the settings to make it easer.

Load up the files in Sketchup and go to Window - Styles. Click on the Select tab (should already be selected) and choose Color Sets in the combo box. Click on one of the colour sets that has a good level of contrasting colour that makes internal and external faces easy to distinguish. The list will show a number of graphics with different options. The larger object in each graphic illustrates the external face colour and the smaller the internal face. Choose the one you like best.

You should then be able to readily distinguish internal and external faces in your objects. Protip: you should not be able to see an internal face, as each of these should be facing inside a solid object. Right-click on each one and select "Reverse Faces." Once you've done each one you can see (there aren't many), export to STL and you should be hot to trot, although I'd recommend using the netfabb cloud service to make sure.

Sorry if this is over-explaining it but it's difficult to pitch to the right level.