Are you using the Fel-Pro double lip RMS? It's intended to seal better on worn crank surfaces. Are you making absolutely sure the seal is facing the right way? Are you making sure not to tear the top half of the RMS when you insert it in the block (the "shoehorn" that comes with the RMS helps with this) I installed a double lip RMS in my 160k truck last year and haven't had any issues since... except with the Dorman oil pan that I bought. That got switched out for a factory piece from a 1995 engine and thrown in the scrap pile where it belongs.
I also used a very, very, VERY small amount of Mopar anaerobic sealer on the chamfered outer edge of the main bearing cap.
Of course, make sure it's not anything else leaking and dripping in that area before you do it again.