There are a few quick questions to ask to be able ot fine tune your answer.
1. Are you exporting a 3D view from Revit?
2. Are you exporting as a 3D solid instead of polymesh?
3. Are you looking at the dwg in a 3D view (Isometric) in MEP? You can type vports make sure that setup says 3D and change the view to any Isometric view to see it in 3D.
Hope this helps you out.