Hi I have just started doing research into the FULLUM family (and FULLER) from Washington Co. PA. I don't know if there is any connection to your Mary Fullum but would love to find out if there is!! I realize how difficult African-American research can be so I guess it is great to have as many people doing it as possible.
We have a Benjamin FULLUM in the 1790 Census in Washington Co. PA seems to have had a few sons one at least was named NELSON FULLUM he is on the 1810 and 1820 census there, he died in 1829 in Washington co. One branch of the family are using the name FULLER whether that was by accident/intentional I don't really know. I'm fascinated by the Mary Fulham/Fullum connection, perhaps a coincidence. Hope to hear from you to compare notes. Kindest regards Dianna from Australia.