This cannot be set for the Slab.
What you may try is Model View Options. You can set Solid for the various Fill Categories there. I don't know whether you are talking about Slab Cover Fills or the fills of the Slab in sections (Cut Fills).
There are options for both in the Model View Options Dialog.
One problem is that it sets the fill for all fills in all elements, not only Slabs.
See if that works for you.
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