For future reference for others: I found the "missing link" - the documentation which links Robert P to his father and his children.
Abraham Johnson/Johnston 1785 - 1853 died without a will and/or there was a legal battle over his land. His land was deeded to his heirs by the court. In the deed book (found by the index), Robert is named as deceased son of Abraham and Robert's heirs (his children) are listed.
The index and deed are found at the WV State Archives at the West Virginia Cultural Center in Charleston, WV. Taylor Co. Index to Deeds. Page 149.