While thinking about how much Marcia reminds me of my mom, I'm wondering if there are/were any of those "trucker plaza" type truck stops with showers and shops and stuff near where she was found. The soot and excellent hygiene point that way, and those places were full of creeps.
My dad was a trucker and my mom and I went with him when I was a kid, and when I was a teenager (and very obviously still a child), the clerks for showers would ask if we would like to shower separate or together. We were both so grossed out, my dad said something like "don't talk to any of these creeps" but the clerk just shrugged and said "just doing my job..." and that was a really nice "upscale" truck stop.
He spent the rest of that trip pointing at his fellow truckers and saying any one of them could be murderers or kidnappers, rapists, etc, and how shady goings on were overlooked and occasionally encouraged. I thought he was over exaggerating to keep me out of trouble, but after being on here I'm thinking maybe it wasn't an exaggeration.
And there was this whole kind of subculture, like pioneers or wild west stuff with an emphasis on selling some romanticized notion of Native American culture. I know there was always turquoise jewelry and whatnots for sale, and leather moccasins (which I wonder if that's why Marcia was barefoot, those never stayed on right) and baskets and other knickknacks sold on site at these places or right off site. I would bet that at some point you might even have been able to pick up a buckskin jacket... I can't say that with any certainty, but its probable I guess.
Not sure if my ramblings can contribute anything that hasn't been thought of before, but ya never know I guess. I've got Marcia on my mind today, I really hope they find with a quickness that (curse words) person who killed her.