They did have a car, The matches they were carrying came from my state of Nebraska and their pictures were recognized at the truck stop although they had no information on the couple.
Things really were different in the 60's & 70's. Just look at the case of Reet Jurvetson - she went missing and her family never filed a missing persons report on her. Young people were joining cults, communes and just dropping out of site.
It doesn't surprise me that no one recognized them. They were found in a remote part of South Carolina. John & Jane Doe's and even murder victims, missing people just weren't publicized outside their location. I only heard about them because I used to read a lot of Detective Magazine back in the day and they were in the magazine. There weren't a lot of true crime books, no internet. Crime wasn't really covered on national news and local news had enough of their own. Even though the matches came from Nebraska, it was in the Western part and I don't think it even made the news here in Omaha.
By the time publicity really came around, a lot of their relatives may have already been dead, and if you were looking for them, where would you start? Since they were traveling around the country, they could have been anywhere. They may have been able to report the car, but since the police didn't know exactly what kind of car they had, that would have been almost useless.
I think DNA will solve this. Just watched the Jane Doe Murders on Oxygen, and like a lot of recent cases, they have a lot less information than this case and have amazing results.