They may have been able to get search warrants once the W's moved or right around that time. Maybe combining that with other information they had, they then were able to obtain warrants at that point. Possible the search is when they actually had enough probable cause to do so and that was around the time they moved.
I do wonder if someone else did get the GMC from them why they did not come forward when Dewine asked for information. Maybe they did not own it at that time or they did give that information but that vehicle could not be tied to the murders at that time. If it was someone other than family that got that vehicle I guess its possible they were not familiar with the case or did not know of Dewine's request.
At that time, investigators said they were “interested in receiving information regarding any interactions, conversations, dealings, or transactions that the public may have had with these individuals, which could be personal, business or otherwise. Specifically, information could include, but is not limited to, information regarding vehicles, firearms and ammunition.”
UPDATED: 4 members of Wagner family facing capital murder charges in Rhoden family murder case - The Highland County Press