Hey there,
I used a pair of profiles in Sketcher, one from the side view, one from the front, and extruded them both into each other using the option to keep only the volume that was common to both of them. This gave me the basic "T" shape of the truck. From there, it was a matter of sweeps and other profiles along with lots of edge blending, gradually adding and subtracting more pieces until I found something I liked.
Since I use NX to do the models, and only use Inventor to do the renderings (I move them from application to application as .STEP203 files), the options NX gets are somewhat different in application but they serve a similar purpose in practice.
Hope this helps.