I have a Lang 48 and start things as you do with a chimney or more of lump charcoal and two or three splits on top of that. Vents fully open, firebox door ajar and the chamber door open for about ten minutes. Close the chamber door and let it go for a few minutes then close the fire box door. Watch the temps and adjust the vents as necessary. When the cooker is up to temp, there is typically just 1 vent open about 1/2 way. I also put a stick or two on top of the fire box to warm up before they go in the fire. The smoke will be different that on a drum, ie more visible, but the flavor from that smoke will be smoother and more subtle. The smoke should still be a pale bluish though.
If by slabs you mean the thin outer slices of a log with a lot of bark and little real wood, that could be your problem. Small splits the size of your forearm and about 13" long work real well for me.