Robert Ross Merrell/Merrill was my grandfather. In my limited research I've never been able to trace further back than John Merrell. However, it appears that the family migrated to Jefferson County from south-central Tennessee. Some old correspondence in my possession is post-marked "Pulaski, Tennessee" and there is still a concentration of Merrell's in that area.
When time permits, I would like to go there and do some searching.
Just as a matter of curiosity, how are you connected?