Hi Caliban,
I've not long been using AC too, and have tried a few ways to do the slabs. Early on I tried placing a fill in the section view to the shape I needed which works OK if you don't have heaps of sections to do. I then tried the Slab Accessory of Aus_Footing which came with AC8.1 which was quite good and created a detailed, customisable model attached to the slab but was difficult to handle the split levels as mentioned, including most garages!
I've most recently been using the Slab_on_ground Patch which is a customisable "object" I found in the Aus_Essential library, once again the library came with AC8.1! Do you have this object library?
I place a slab object at floor level at the thickness of the main floor and extending to the outside of the ext walls, then place this patch on each end which gives the edge beam and footing. When the materials are set to match the slab, they blend to appear as one solid object.
One main helpful thing I have learned about ArchiCAD is: The more you model, the less you have to draw!!