Thomas lived in the same township in the 1830 and 1840 Census and then is gone in the 1850 Census. Probably died of old age. No other
Grahams had the same spelling either they were all spelled
Grayham not
Graham like Thomas and Robert. You might want to put a post on the
Madison County page and see if you can get a birth, marriage or other record to confirm relations. Someone might have an index that is not available on ancestry yet. YOu may also check out Findagrave.com It may have some good information for you to find there.
Just a few suggestions.