He stayed. He married Helen in its first episode. Maybe he left later into it. I haven't seen R. F. D. Just the final T. A. G. S. episode of the same name that introduced it. The spinoff starts with Ken Barry(Sam Jones) having the Vinchetti family his friend Mario, Mario's sister Sophia and their dad from Italy help work his farm.
I hate that too when it's also done with The Andy Griffith Show. People say "The Andy Griffin Show" when mentioning the show or "Andy Griffin" when talking about him.