In 1845 the area where the McDowells lived, Farmer Twp. became part of Defiance Co. ,OH.You might want to post a message there to locate any remaining descendents.
I did find the John McDowell family in the 1840 Williams Co., Farmer Twp census. It included 1 male under 5; 2 males 5-10 years; 1 male 10 to 15 years; 1 male 40-50 years; 1 female under 5; 1 femaile 5-10 years; 1 female 40-50 years.
The only other mention of McDowells I could find in the books I have on Williams Co. I have was 2 marriages: Horatio C. McDowell to Rebecca F. Manon 13 Nov 1874 Thomas J. McDowell to Hannah Tingley16 Apr 1871