DWGs created in AutoCAD Plant 3D cannot be displayed as Solid.
Pipes are displayed in wireframe
Plant3d object enabler is not supported in RealDWG.
Since Plant3d is an Autodesk’s product, it needs to be supported by Autodesk.
We did a test on AutoCAD 2019 by installing plant3d OE, that confirmed this OE did not work on RealDWG. Here is a default display in AutoCAD when enabled.
Object Enablers for Plant 3D DWGs