I found a 1900 census with a Fullerton family in DuBois, Clearfield, PA.
"United States Census, 1900," database with images, FamilySearch (https://familysearch.org/pal:/MM9.3.1/TH-267-11867-132185-50?cc=1325221 : 5 August 2014), Pennsylvania > Clearfield > ED 70 Dubois borough Ward 1 > image 13 of 70; citing NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, n.d.).
Here's what it says:
At this point, if John Leroy Fullerton was their son, he'd be 4 years old... and likely in the census. Also says that Clarra and Harry wed in 1880, have been married for 20 years.
If Harry B Fullerton is the same as Harrison B Fullerton, his death certificate is on Ancestry. He died April 26, 1915 in... Lawrence? Township in Clearfield county of PA. Death certificate is 34074. If so, I do believe this is their Find a Grave page.
There is also a tree on Ancestry that references a book currently not available online that maybe your library could get in. "The Fullertons of North America, Volume 2, Part C" by Gordon W. Fullerton, Jr.. 8th Edition, Published by Polynesian Dynamics, Honolulu, Hawaii, 1999. It's cited as a source for the marriage. Said family tree also does NOT list a John Leroy Fullerton as one of the children.
EDIT: If you want to really find out who John LeRoy Fullerton is, I'd suggest getting one of your mother's brother's sons - if any exist - to do a Y-DNA test and possibly an autosomal DNA test. Emphasis on the Y-DNA. Y-DNA is passed from father to son.