They don't appear to be meth-heads: scrawny, pock-marked skin, missing teeth, crazed eyes. They graduated, which is an accomplishment. Sherrill maintained a tidy home. Meth addicts are often so consumed with getting a fix and living in a haze they have little time for real life things like keeping a job, paying bills, going to school, personal hygiene, respectability. When they come down they're paranoid and violent. Heroin, maybe. But even those get to looking and behaving badly before long. So I think there would have been signs.
My husband is a pastor, we are surrounded by the **** and drug culture here. Shootings all the time. Robberies. Bad drug deals. Overdoses. But nothing like three women vanishing from their home in the middle of the night. I may be in the minority, but it seems an awful lot like a sexual motive to me.