I don't know if this has been said before, but maybe the girl left her home with her (possibly single) mother behind. The mother says 'good riddance' when her daughter leaves. Maybe she doesn't like her, maybe she's glad to go and see that 'b* father of hers in prison'. When the girl doesn't come back, the mother is glad to see the back of her.
As for why she runs away? Maybe her mother's significant other is abusing her. Maybe the girl's mother knows. Maybe the girl's told her friends. When she leaves in 1980, the girl's friends think that she's gone to live with someone else. Maybe the dad that the family don't talk about, who they don't know is in prison, perhaps without the kid listed in his information because nobody knows about her. Perhaps not even dad himself.
The girl leaves the diner. She comes across a stranger, maybe. But possibly even mom's significant other. There's an argument. Girl says 'I'm going to say you abused me' or perhaps blackmails him. She's young; she's not thinking clearly. He gets angry and because he doesn't want to take his clothes off right now and actually 'teach her a lesson', he uses something to assault her (note: perhaps he was impotent and used an object frequently). Then he kills her and dumps her.
Maybe mom knows about the murder. Maybe she doesn't.
Either way, I think the girl had a terrible family life. But chances are she told someone about this and middle-aged women should be checking their high school yearbooks.
Your story is as plausible as any other, however, could've been an aunt and uncle.. Could've been a foster home, any number if scenarios. I don't think she was killed by someone who knew her. I think that she was killed by someone who has killed before her and most likely after her. Only way a person like that would stop is if they were incarcerated or dead. This was not a rape gone wrong, this was a brutal attack of a defenseless girl. This was swift and close to the truck stop.... My thoughts is she was raped at the truck stop in the cab of a truck or raped in a vehicle, then taken to a second location to be offended again and it got bad. We don't even know if she was or wasn't a working girl. Desperate people do desperate things. Perhaps she picked the wrong John... I don't want to believe she was a working girl but who knows... That's the problem. We don't know anything . If she came from the south of 45 (hitching post is on 45) she wouldn't have been that far north on 45, Ellis unit is east. It's actually closer to Trinity.
Let me ask this...
Who better to than a prison guard in his TDCJ about a ride to a prison?
She wasn't going to go north or south on 45... She needed to go way east. Odds are whoever gave her a ride to the hitching post was traveling on 45 North or South... We didn't have Google maps back then. My guess she came from the south of Texas, got on i45 after traveling up through Galveston or Victoria.
If a girl makes up where she is from, Rock Port wouldn't be a rambled out answer. She would've said Houston, Corpus, Dallas, Galveston. No doubt she is from Rock Port. Population 8000. I'm sure much smaller in 1980. Somebody in Rock Port in 1980 knew this girl.