The 1850 Federal Census lists a David Knap 19 RI living in Cranston, RI, with the family of Samuel Larkin. He is a laborer.
He may be the David Knapp age 22 who enlisted in the army 1 March 1852. It says he was born in Johnston, is a farmer, and is 5 ft. 6 inches tall with a ruddy complexion, blue eyes, and brown hair.
In the 1860 Federal Census, the David and family are living at Montville, New London Co., CT. They are with Daniel Abel and family.