Continuity mistake: When Rusty Nail has the car pinned up against the tree, his truck breaks the windows. The driver's side window is completely broken in one shot, however when Rusty leaves, the window is shattered but largely intact.
Continuity mistake: During the cornfield scene, there is at least one point where the spotlight on the truck is off, but then on again.
Answer: For Fuller and Lewis, the best assumption is that he never actually left the first motel. He stayed at or near the motel, followed them to the police station, and then continued following them up untill the point where he makes his presence to them known again. Or he also could have just parked his rig somewhere far down the road and waited for their car to pass him, and at which point he would start following them. And for Charlotte, he probably just followed Fuller and Lewis all the way to the college, saw Charlotte, and then kidnapped her while she was leaving.