In 1800 there were only 3 Knapps in census:

David Knapp
Mayfield, Montgomery, New York 11

Joseph Knapp
Florida, Montgomery, New York 6

Solomon Knapp
Mayfield, Montgomery, New York 4


IN 1810 there was only one Knapp left in Montgomery County,

Solomon Knapp
Mayfield, Montgomery, New York 10


I guess ya can start there. Hopefully one of those 3 will be Elizabeths family.