The Export to 2D DWG and 3D DWG in SU returns two different results as pbacot said.
2D DWG - all elements are moved to AC Layer 0
3D DWG - all elements follow SU layering names.
The 2D Export is not creating the desired effect, but it has to be some reason why the 2D DWG Export looks like 2D DXF Export.
Many years ago I used to export 3D DWG files into *.wmf and 'Import' them back through AC's Insert > Windows Metafile... to get 2D representation of roofs, elevations or sections and apply colours or hatch.
Step 1
From SU Export to 3D DWG > and open in AC
Step 2
In AC Export to *.WMF
Step 3
In AC Insert > Windows Metafile... (AC's top menu)
Change the view to get plans or elevations.
Use AC command HIDE to hide elements in the background and then export to *.WMF
Unearthed, I don't know if the 2D is good enough for you.
One thing's for sure, it keeps the SU layering in the AC 2D drawing π
PS. Windows Metafile - was the useful vector based format for Adobe Illustrator, Corel Draw and few other graphical packages, which worked better than the DXF or DWG vectors when imported.
Regards