I would think that with the age of the furnace, a bearing would not go bad that quick.
I believe that the furnaces us a single motor for both the air blower and the combustion air too. These are held in place by "allen" screws, and wondering if one screw got loose causing blower wheel to move and hit side of enclosure that caused noise.
I found out a lot about RV furnaces when a Mud Dabber got into the combustion blower section of the furnace, and the nest came loose and jammed up the blowers. On a cold morning I had to take the furnace out and disassemble it on the table to remove the nest.
Furnace had always vibrated a lot before I made repair, and after that it ran nice and smooth. The nest had been on the blower wheel before it came loose, which made it go off balance and caused the vibration.