@ggc: @paperdigits has been quicker than me to give the answer, anywhay what he wrote is 100% correct.
Let me just add that once the AppImage is extracted (contents actually go into a squashfs-root folder instead of AppDir as far as I remember) and you have modified some files, you have to run RT by launching the AppRun script in the extracted AppImage. Runninf the RT executable directly will not work, as the AppRun script sets several environment variables in order to let the RT executable find the bundled libraries and configuration files.