I'm not sure which Joseph Willis you are looking for, but the one that died in 1809 in Mercer Co., KY was in the French and Indian War not the Revolution. If you are looking for his son, Joseph who d. in 1818 in Mercer Co., KY he and his brother fought with George Rogers Clark. They were in KY by 1779. They are listed in GEORGE ROGERS CLARK AND HIS MEN MILITARY RECORDS 1778-1784, by Margarey Heberling Harding pulb., by KY Historical Society 1981. The brothers are on p. 56 & 185. Joseph's last name is illegable as well as Wm. B. Rice whoes name is directly above his. John Willis is also on that list but he is indexed as John Willie.
I descend from Joseph Willis' daughter who m. John Hale
I hope this info is helpful.
Arlene (Hale) Brachman