He may have stashed another vehicle within walking distance before he started his killing spree. He may, in fact, have multiple vehicles, weapons and supplies in various locations. He seems to have planned this out for quite a while.
If he has a buddy helping him, the buddy could have driven the truck there, gotten out, set the fire, and walked to where he had another vehicle parked.
They’re dealing with a soldier. One big principle of war is to confuse and fake out the enemy as to your location and objective. http://www.huffingtonpost.com/2012/11/19/difference-between-sweet-