Have you looked for a death certificate? I saw that an Isaac Tillman lived beside him and Julia in 1880. But is there any chance that Reuben could have went by another name? This is the entry for the household of Isaac Tilman:

1870 Census Saluda Division, Edgefield, SC

Isaac Tilman 48
Martha 40
? (male) 17
George 12
? (male) 10
Bryant 8

It seems as if the oldest male may be Reuben, but be aware, he could've had a nickname in his younger days. (Can't tell the individual letters, but I know that it's short.)

Hope this helps!! Hope this is your fam!!!!

Antonio