Mike, my Ancestry.com tree may be one of those nine trees you found with Nancy Dowling in it. When you posted about this the other day, I did try to search for some documentation to prove that she was/was not Jabez's daughter and I could not find anything. The book on the Dowlings, A Dowling Family of the South, (pages 3-4) has Nancy as Jabez's daughter but does not source that relationship. This may be where I originally got the information I put in my tree, as did those who set up the other Ancestry trees.
I am descended from Robert Dowling also (his son John.) One of the things I wanted to do was look at Thru-Lines to see if I had any DNA matches with descendants from his son William, which might include his son Jabez and Jabez's alleged daughter Nancy in their lineage, but I have not been able to access Thru-Lines. (The message I get says my linked tree is private, but it's not.)
I wish I could be more help in providing proof.