According to my research, ELIZABETH DANCY PETTUS, born 11 November 1781 in Charlotte County, Virginia, married her neighbor WILLIAM GILLIAM in Charlotte on 17 July 1800. Elizabeth was the last of ten known children of John Pettus of Avondale, Charlotte County, Virginia, and his wife Susannah [--?--]. William was the son of James Gilliam of Charlotte County and his wife Martha. William and Elizabeth migrated to Williamson County, Tennessee. I did not have any information that they were the parents of Allen Pettus Gilliam, but based upon the information in your message, it seems that they were.
Elizabeth Dancy Pettus was identified as the daughter of John Pettus, deceased, when she selected her guardian. John died in 1799.