I think your William Marshall might be the son of Samuel Marshall. His Brunswick Co. will, dated 1815, names his wife as Rebecca. In the will he names some of his older children (by a first - unknown -wife), and refers to unnamed younger children (not yet 21 years old). Your Paul and Saul MIGHT be among those.
Samuel Marshall, his father, earlier lived in Brunswick, but later moved to Halifax and Warren County, NC, where he died ca. 1785. Samuel was the brother of my John Marshall.
If any of this sounds promising contact me offline email@example.com
I have a huge file on this Marshall family that might help you more, and it includes a full transcription of William Marshall's 1815 Brunswick Co. will.