I am descended from your Sills family.It's quite a while since I worked on them.I do remember that Isham Sills operated a ferry on the Cumberland River, and I know that his will was a noncupative will.It gave almost no information, but noncupative wills are made under great duress, being given on one's deathbed, usually oral only, and repeated by the witnesses at the time of probate.
The 1830 census shows Isham to be between 60 and 70 years old, so he must have been born between 1760 and 1770, and the estimate of 1750 may just be too early.
His father, of course, was Isham Sills, Sr., and Isham Sr's father was Lambert Sills, who married Elizabeth Brewer, according to an email I received some time ago from Ken Garland.
Also, I think there was probably a mis-reading of the will naming a son Samuel.I suspect it was Lemuel, instead.Lemuel is not so common a name, and the capital L and capital S of handwriting in those days was quite similar.
I'm actually thinking my line is through William Sills.I have not found a list of William's children, but I believe that two of them were the Lemuel Sills who married a Wallace, and lived in Trigg County until after 1750, and Permelia Anna Sills who married Benjamin Patrick Hicks about 1838.I don't have positive proof, however, so I cannot claim documentation for this.I don't have it.At least, not yet.