Medial surface extraction is a hard problem. If you had the information in your CAD system then export it from there instead of trying to reconstruct from an exported surface mesh.
Anyway, if you don’t have any other option, you can use vtkvmtkPolyDataCenterlines in the VTK-based VMTK toolkit to get medial surface (returned by GetVoronoiDiagram). I think the Voronoi polydata already contains thickness values in a point scalar.
Alternatively, you can convert the polydata to a fine-resolution binary image using polydata to stencil filter, then use ITK filters to get the medial surface, compute a distance map from the skeleton, and sample that with a VTK probe filter. See a complete example for this approach here: https://discourse.slicer.org/t/how-to-analyze-the-thickness-of-the-model/2735