I hope this reaches you as I know your post was from several years ago.This is my Thomason line too and I would love to share information with you.
How are you connected to Thomas Jefferson Thomason?His son, Jessie Harrison Thomason, was my great-grandfather and Jesse's son, Thomas Benjamin Thomason, was my grandfather.
I believe Jonathan's father may have been Edmund Thomason and his brother may have been David Thomason.All three families are listed on, I believe it is the 1855 Illinois State Census, as living right next door to each other in Cumberland County.Edmund was in White County, Illinois in 1830 and Jonathan was married there in 1837.
Years ago, I found a will from Smith County, TN for George Thomason at the Genealogical Library in Salt Lake City in which Edmund from White Co., Ill was named as a child and heir.I didn't work on my genealogy for many years while a single mom and haven't been able to find my copy of it since my last move, though it is probably in a box somewhere, but I used to have a copy of that will.
I have not found where Jonathan, David nor Edmund were buried though. Have you had any luck with that?I was born in Mattoon, IL and still have family in the Charleston & Mattoon area. I don't get back there often enough though.But I would love to compare notes with you.Please email me. Thanks!