AJT: We have a Jabsco Quiet Flush on freshwater, a different model from yours but perhaps similar in some respects. I tried to replace the shaft seal
last Spring, but that didn't stop the slight leak that had developed. The unit had also been getting noisier over the last of the previous season, and while occasional dollops of cooking
oil did help that symptom until the leak appeared, replacing the shaft seal
didn't solve the noise
After that, the nice Jabsco folks allowed as how I'd probably gotten my money's worth from the original macerator... so I then replaced the whole motor/pump assembly, and that fixed it. Relatively inexpensive fix, relatively easy to perform.
Having tried it once. I wouldn't bother to try replacing a shaft seal again; I'd go straight to the whole motor/pump assembly. (Assuming intent to repair the existing unit.)
You can get the manual and parts
illustration at: http://www.xylemflowcontrol.com/marine-and-rv/