Just to kind of echo what others have said, most likely this was a two-person stalker job. I hate to sound like I'm stereotyping, but anyone who knows the area knows that there's no scene more common than two guys driving around in a pick up truck. I say it's probably a stalking case gone violent because the perps knew exactly where to find her, where to put her car to make people think she was in that area. I'm guessing she might have been followed from her house.
This young girl was a celebrity, maybe only in the Internet, but a self-style celebrity all the same, and celebrities have haters/stalkers. The area where she's from, and near the gas station, is a very low population, ethnically mixed, low-income area and putting all of her personal information about there, her naivete made her a sitting duck. This young woman's online life was a ticking time bomb and it was only a matter of time before it landed on her family's doorstep.
As to where she is, her body could be in the woods or it could be somewhere else. Many people in the area are connected to people in other rural areas; there are isolated counties in every direction.
I do think the FBI and LE aren't showing all their cards, they can't, because people in this type of rural community are all very well connected. They cover for each other.
Regarding the two strange Mexicans: the area has some agricultural businesses that employ migrant workers, so take those two Mexicans and multiply them by about 1000. There are a lot of them in the area, generally no trouble to the community. It mostly the drugs, unemployment and the emotionally disturbed that cause the biggest trouble in the area.
Regarding the question about a body of water: there is a lake not too far from the gas station where she was last seen, but from what I understand, it's fairly developed with homes, families camping and hobby fishermen, so it's not isolated.
