First, I would make sure it is the RMS. Take off the access plate for the clutch/flywheel. You should be able to see the oil path if it is the RMS.
Check your pan gasket, the 4 piece are prone to leak.
If you do have to replace the seal, I would check the clutch and flywheel as well as the T.O. bearing and release bearing. You will have all but the T.O bearing removed anyway.
Until you fix this, make sure you keep enough oil in the system. I was driving from Denver to Phoenix in 1982 in a 73 FJ40 with a leak and ran out of money. I was sure I could make that last 200 miles. Nope, blew a rod through the block 93 miles out of Phoenix near Payson.
The leak was on the head cover though. One of the screw mounts on top had cracked. Found out about 300 miles out of Denver. Put oil in at every stop the rest of the way.
These engines are tough as long as you keep them lubricated. That experience has made oil leaks high prioroties for me. When I got my 77, there was a slow leak from the oil pan. First thing I did was replace the oil pan gasket.
Besides all the other things mentioned above, one other likely leak is the cam plug. You can see it with the flywheel off and it is partially obscured by the bellhousing. You need to remove the bellhousing to replace it. The clues that rear main is leaking is that it will leave radial oil streaks on the front facing side of the flywheel. The cam plug leak won't leave these streaks and instead tends to drip off of the passenger rear main cap bolt while the engine is idleing and the truck is not moving. I have seen more cam plugs leaking than rear main seals.