I've known about this case for at least 25 years. And I live in Canada! I think the mystery moved beyond the border because 1) there was a suggestion they may have been French Canadian or a least resided in Quebec and 2) it has all the earmarks of a great mystery.
And to be fair to the parents and/or family members of missing persons back in the 60s and 70s, there was really no way to register a missing person. I mean, the logistics of when they went missing matters. People traveled, a lot, during that time, sporadically getting in touch with family members through postcards and collect calls. So just because the last time you spoke or got a postcard from your child in Wisconsin doesn't mean that's where they went missing.
It used to be a joke that, so a parent wouldn't have to pay for a collect call from, let's say California or England, they'd give a code name so that when the operator asked if you'd accept a collect call from Maggie Muggs or some other fake name, they wouldn't accept the call but were assured that their child had arrived at their destination.
Also, having matches from a truck stop doesn't mean they had a vehicle. It just might mean that's where they were dropped off by someone who picked them up hitchhiking.