Yeah, a trucker would've likely gotten on the interstate rather than continued south. Unless it is a local route driver, like a farm/grain truck or something.
In any case, it seems like they have a handle on what probably occurred. Heck, they may even know precisely which vehicle and who was driving it. But without a witness corroborating that something suspicious occurred with the vehicle in question (slow moving, circling around, etc.) then a warrant is unlikely. So that may be all they are looking for in order for this case to blow wide open.
JMO.