Since DM wanted the truck and the whole part about driving around with a dead person in the vehicle makes me believe that either a, they took the gun for threatening purposes (like, give me the truck, or I'll kill you), or b they were always going to kill Tim, but perhaps not in the truck. Maybe they were going to kidnap him WITH the truck and murder him at the farm. Makes more sense than shooting someone in the vehicle you want/are currently driving.
A speculation on my part only--perhaps when they slowed at Bobcat, TB grabbed the key and attempted to flea, a frantic DM shot at him to stop him, bloodying the truck, they chased him, finished him off and wrapped him in a tarp throwing him in the back covering with another tarp. So as you were saying, shooting inside the truck was not the plan. But I think, unless DM is very poor at directions, if the murder was to be at the farm, he would have driven north to the farm. So many anomolies. IMO