I don't think there is a disassembly or source listing. However there is a package somewhere that is labeled as RSC-Forth source, it appears to be a commented disassembly that has been ported to a Mitsubishi (6502 compatible) micro-controller. It has some things missing, like the disk routines and the serial code has been altered, but it is still quite useful. I can't find a link to it right now, but I have a copy on my computer that I can upload tomorrow.
Edit: Found a link
http://www.4e4th.com/RSC_Forth_V1.7.zip