I'm pretty sure, you got it right that Doss comes from her grandfather's family.
Her stepfather's name was McGee. This website says she might have used her stepfather's name:
http://www.charleyproject.org/cases/d/doss_kimberly_rae.html
I'm pretty certain this is her parent's divorce record:
https://familysearch.org/ark:/61903/1:1:VK45-3N2
A public family tree on Ancestry lists Kimberly's mother as being married to a man by that name and having an (unnamed) daughter with him. That family tree says he died in Tampa, FL but the tree does not have a date of death. I have searched and I cannot find a death record. Therefore, I'm going to refrain from saying his name. Let's go by his initials: CAL. CAL apparently divorced Kimberly's mother while Kimberly was an infant. My best guess is that CAL was not involved in her life and she may not have even been given his surname. I was looking for evidence she was raised by her grandparents to explain that last name but found none. Her mom remarried (McGee) in 1974 and had a baby boy right away. Kimberly would have been 11 when her mom remarried.
Lastly--this is just a theory of mine, but it's possible Kim was adopted by her stepfather and took his name at that point. Original BC would have been reissued in an adoption and the reissued BC would often be kept private and not found on ancestry websites. All this info is relevant, IMO, because anyone looking for old newspaper clippings about Kimberly's disappearance needs to know which name to look for her under. There is so little info about her disappearance online I think the only thing we have to go on is a physical resemblance and that story about "a" Kimberly Doss showing up in Hollywood and Sugarland Texas. I can't even find out more about that lead in Sugarland and what became of it.