Hi, I am also researching my Fothergill family.I have heard that my family is also related to the Dr. John Fothergill, but I have not made the connection as yet.
You might also look at the different spellings.I have found it spelled as Fathergill, Featheringill and Fotheringham; changing from one generation to the next -- and also due to census taker spellings.
I have traced my line to: John Featheringill recorded on the 'Continental' Census 1742 in Frederick County Virginia.
I also have found a John Fotheringham married June 1747 in PA, to Margaret Shoemaker.I am still just piecing things together, as you are.
Maybe we are related on our lines?As far as John Fothergills ~~~There are a lot!My dad is a John, my brother, my brother's son and many others back through the history of the Fothergills!!Makes it hard to keep them straight and to locate the correct ones!:->Part of the fun of our search!