I just got my Z71 and just as I figured, the cargo storage is much less than my previous Armada. Like the previous poster said, it's deepest toward the interior and shallows slightly as it moves toward the liftgate. I was able to put long jumper cables in it and the depth is just enough to fit multiple trailer hitches/balls.
As for the small electric air compressor and tow strap I also carry, there's a removable side panel on the back left where the jack is stored. It has enough room there to fit those items plus a spare can of fix-a-flat and a few road flares I threw in as well.
Depending on the dimensions of your toolkit that may an option for you.