If it was your rear main then you should be seeing some oil around the junction of the engine and transmission. That area is pretty easy to find when underneath (splash pan removed of course). However, the RMS is not the rear end of the oil pan, thats a part of the pan gasket itself. The RMS is a small ring that seals where the rear end of the crank exits the engine block, you cannot see this area with the tranmission in place. There is a little drain hole in the bottom of the transmission bell housing, where it meets the engine block. Oil will probably be dripping from there and then getting sprayed by the wind all over the adjacent area of the engine/transmission bottom ends. If there is no oil coming from that drain hole or there's none on the seam between the transmission and the motor I'd agree with you and say the RMS is probably not the culprit. There are lots of other areas higher up on the motor that oil may leak from and then run down the block and get sprayed around by the wind. It's best to look at all the usual suspect seals on these cars and see if they look unusually wet or deteriorated.