Hi etsmc, I had a look to your STLs (I will send you an email privately with the pictures of your CAD model showing the critical areas). Here are my remarks:
1) The original geometry is critical: you used some strange variable curvature radius and there are many cusps if you cut the models into sections
2) It is ok to use MeshLab or Paraview to decimate the triangles
3) If the native software (SketchUp) is not able to generate a closed STL, there is no reason to think that NetFab can do it (btw: NetFabb has been acquired by Autodesk too?)
NetFabb actually generates a closed STL, but it "guesses" too much around the critical areas, this is an example:
What you have to do, is simply to design a correct and clean geometry with SketchUp or with other software (a more "honest" CAD tool than ShetchUp would have rejected that geometry) in order to not use NetFabb at all.
(You would not need to use NetFabb with a clean geoemtry, but 9 times over 10, it would be better to use NetFabb before decimating the mesh with MeshLab). More details and pictures by email