Mike, I'm using V11.0.1. What version are you using? Not sure if V11.5 would help but some of the posts I've been reading have discouraged me from upgrading just yet as I've several projects on at the moment.
Using a combination of grouping the hardscape object and then converting to 3D polygons and applying the class got me a 3D surface that rendered but I cant seem to do it again! And of course I lost all the 2D flagstone pattern.
Also, drawing a hardscape with the Draw 3D option deselected and then Converting to Group followed by Converting to 3D Polygons gets a 3D black and white object with the flagstone pattern, but no texture at all???
So at the moment it looks like the only way is to generate two drawings - one 2D and one 3D - not very efficient, and a little haphazard!