I have never seen one where the jack tube could be lubed in any way. The heads can but even there, be careful, as the grease can melt into runny liquid and get into the motor head very easily. The best jack I ever had was an old Reese, no longer made but quiet, powerful, fast and the motor was above the tube, not offset to the side with the shaft subject to water from above. I still have one on my '74 Argosy, working after 40 years.
I am afraid to say that I think you just use them until the fail, then replace. You probably should have a manual crank, although if the main tube is the problem, the crank may not work better than the motor does.
Gears can be replaced from the motor to the tube drive in most jacks, along with switches, but not much else that I know of. (depends on the specific jack of course).