I also assumed that the Thomas Marcus Hulings in question was the son of David Watts Hulings. The Dauphin Co. Genealogy Transcription Project at firstname.lastname@example.org also mentions this Thomas as their son: "he died leaving children, Thomas, Maria, Ellen, Mary, and Lizzie. Thomas married a daughter of General Thomas, of Washington D. C.; was a colonel in the Civi War, and killed in the battle of the Wilderness." This is apparently in error, because according to Historical Register and Dictionary of the U.S. Army 1789-1903, Vol. 2, Col. Hulings died at Spotsylvania 10 May 1864. He commanded the 49th Pennsylvania Infantry and was at the battle of Gettysburg. I did not have a birthdate for him, except "about 1816" from an LDS IGI file.
His daughter was Ella "Elizabeth" Hulings, b. 1862 in Rockville, MD, my husband's great-grandmother. She married Nicholas Dorsey Offutt II in Rockville on 26 Oct.1881. She did not have any siblings and I have not been able to find out anything about her mother. She attended a boarding school somewhere in the area and was in possession of her father's portrait and medals. A Civil War pension record I found on ancestry.com says his widow was "Mary B. Hulings" and another record gives a guardian as "Mary B. Vickery."
I have more information about descendants. Feel free to contact me.