under Options you should set the export settings to double faced.

From recent experiments, the double face option should be avoided when you export to OBJ format from Sketchup if you want to reuse your models in Blender. Such OBJ models can be imported correctly in Sweet Home 3D but Blender doesn't always import them correctly, reorienting some faces. Probably a bug in Blender OBJ import handler.
The double face option isn't a problem when the exported format is DAE/Collada.