Ok, i was wrong in my previous post, like potatis_invalid already mentioned, you need to make the front end of your truck face west, as seen from the top view in this screenshot:
Ingame the truck will face the first path_track.
Also, its best that you put the origin and the first path_track in the exact center of the truck. To get the exact center, make a temporary brush and cover the entire truck with it. Then select it and hit CTRL+E, this will make the grid viewports center on the brush. Now look for the X that marks the center of the brush, since the brush is the same size as the truck, this X marks the location where you should put the ORIGIN brush and first path_track.
Don't forget to raise the other path_tracks to the same height as the first one, otherwise your truck will partially dissapear in the ground.