The correct R/T for an RTO is 'birdseed 121 stopping', you may add 'engine fire etc' to that if relevent. I might also say 'request full assistance' if I needed it.
I stopped the other day for a config. warning, and said 'xxx stopping, no assistance required' which I think atc were grateful for as it saves them asking. We wouldn't have wanted him to hit the crash button for that!
Hope this helps.