Sounds like we are definitely cousins. Too bad there is still so much unknown about our early Thomas ancestors.
I note that there was a son, aged 15-20, living with Ross Thomas in the 1840 census. Perhaps this was your William Thomas, but it wouldn't explain why he was living with his grandfather John 10 years later. In 1850 Ross Jr. had already moved to Van Buren County, Iowa as he shows in the 1848 census there, along with my g-g-grandfather Isaiah Thomas.
At this time, I am pretty much convinced that Isaiah was a son of Richard Thomas. No absolute proof, just rather oblique references in some old letters.
If you have an interest, most of my information on the Thomas & Gray families is posted on the following FTM web site: