My money is on the issue of jurisdiction for potential charges of homicide, accessory to homicide, and accessory after the fact, for why it's taken so long to decide to bring the truck back to MI.
It's not just which state, but which county within state, in which crimes occur that determines jurisdiction. First, LE has to determine which crimes occurred, then where they occurred, then the prosecutors and their investigators in those locations have to weigh in, then if multiple charges are potentially going to be brought with differing jurisdictions, the prosecutors from those locations have to decide who's going to bring which charges and therefore needs custody of the related evidence, and if one item contains evidence related to crimes in differing jurisdictions decisions have to be made on what to do about that, and then the prosecutors may decide that one county/state will bring a number of charges even if jurisdiction would normally go to a different jurisdiction.
Jurisdiction is the only thing of sufficient complexity I can think of that would warrant the length of time it's taken. Chain of custody is secondary to that, because custody is dependent on the decisions on who's going to bring which charges.
The best thing to do until all those decisions are made is to leave the truck right in the custody of the responsible agency where the truck was found.
I think I may have broken my record for length of a run on sentence above.