If all other fish have been feeding well and behaving seemingly normal, then I'd suppose that your elevated ammonia has NOT been at 1 ppm or some such throughout this ordeal (I mean that 1 ppm measure is not representative of what's going on) but rather 0.25 ppm or some such.
Fish cannot really adjust to 1 ppm ammonia.
At 0.25 ppm ammonia level, fish have some capacity to adjust and still live ok but not long term... and the long term will vary for different fish. Sooner or later their immune system will break down. No question about that. 0.25 ppm ammonia is still a constant stress. (Perhaps unless your water was really acidic (low pH of say 5-6) at which condition the ammonia is naturally converted into the non-toxic ammonium form, but this is not your case).
I commend you for listening to us but remember that ultimately you are the only one in position to see everything and judge it.
There is always the weakest fish / link. That RTC must be the first to go unless you caught it in time and detoxified the ammonia (completely) in time and you luck out.
Or like you say, perhaps this fish has not been viable from the start. Still you have been learning a lot hopefully from this experience.