Not that I'm trying to excuse Cammie's behaviour in any way (I think she is acting like a complete entitled brat), I can see why the OP is trying to steer a middle course. Cammie is only 18 - yes, that's legally an adult, but it's still very young. And if it's the case that Cammie has indeed come from a dysfunctional family, I'd wager that much of her behaviour stems from simply trying to be tough and survive the best way she knows how. On a certain level, it must be incredibly scary to be 18, with no parental support, no finances, trying to make your own way in the world.
With all that said, I agree that the OP should focus on getting Cammie out of the house sooner rather than later. Personally, I'd be contacting her relative in North Carolina, and seeing if they were willing to take her in. And if you can afford it, I'd also offer to pay for Cammie's bus fare - just to get her off my plate.
(And I'd also Google tenancy laws in Arkansas, just so you have a clear picture of what your rights are, and whether Cammie could be considered a tenant - I'd hate for you to wind up in a situation where Cammie - due to the length of time she's been with you - is legally a tenant, with all the accompanying rights, etc)