my turn.. sps crash..

[Snappy]

I got back yesterday to find the RTN is very slowly progressing on all my sps.

Bummer however your discription sounds like STN not RTN

RTN - rapid tissue necrosis
STN - slow tissue necrosis

STN can usually be stopped by fragging or even some glue around the affected areas with some covering a small amount of flesh stopping it from progressing.
That said STN is generally how reefs are made in nature. The bottom dies off and the tops grow. If it isn't going from the bottom upward you have problems in your system and need to do some tests and likely water changes ASAP.
Good luck.