When toner is on the back it means the toner has spilled internally, probably on some rollers that will need to be cleaned.
If the fuser was an issue the toner would smear off (not hot enough)on your fingers when you touched the output, or flake off (too hot).
Ghosting would be the belt cleaner
I have a workcentre 7125 that seems to have some problem fusing the ink to the paper
I have replaced the transfer belt
The black ink drum
the cyan drum
black ink toner
the fuser unit
Toner "dust" appears on the back of each printed or copied page and every 2nd or thrid print has a ghosted copy on the bottom of the page.
Any help is appreciated.