If you are working with shape-edited slabs in Revit, it can be difficult to coordinate with a civil engineer who is working in Civil 3D. It is fairly easy to export a Toposurface to a mesh, but solids are an entirely different matter.
Even if you do choose to export your solids as meshes, Revit will export the entire solid as individual mesh faces. This is not very useful in Civil 3D, since there is no way to isolate just the top or bottom faces.
But, of course, this can be done with Dynamo. And I have created a custom node that can extract the top and bottom surfaces of any element.