Were you restricted to the vertical composition? Cars (and trucks) are very horizontal things, and it's much easier to fill a horizontal frame. The last one is starting to say something, but you need to find a better angle, and having a silver truck didn't help. If it were yellow, red or blue, there would be no question about it being the visual center, but silver just recedes into the background.
Try to think about layering, and managing the foreground, middleground and background. Again the third image is approaching something interesting, because you've got some foreground elements, and the truck in the background, but you're not managing them well enough. Everything needs to be intentional. Find some more interesting foliage, or a good rock or something interesting for the foreground.
Also, try getting down lower, you're shooting very much at eye-level. It doesn't project any of that sense of dominance or toughness you see in 4x4 ads.
Exhibit A:
http://www.fordvehicles.com/f150raptor/
It's a composite, but the principles are the same. Camera low to the ground, wide angle to exaggerate the size of the vehicle, good action, foreground/background control, etc.
Exhibit B:
http://www.beyond.ca/tag/ford-raptor
Practical, not composite. Notice the foreground, you can see how low the camera is. The shrubbery gives a little bit of something, so the foreground isn't totally empty, and they don't distract from the main center of visual interest (the truck). This approach is a little bit different, as they used a long lens (probably 70-200) and got farther away. This gave them really shallow DoF (but they stopped down to get the whole truck sharp), and flattened everything out. The truck clearly dominates the frame, and there's great action.