Here's a shock--ML RBI leader Miguel Tejada had the most opportunities with runners on base in the majors. Tejada easily saw the most runners on 3rd (120) in the majors, which no doubt helped him considerably. Still, Tejada had one of the better conversion percentages (.212) in baseball, so he was doing plenty to help himself.
Compare the Opportunities by Batter Report to the list of RBI leaders and you'll see a pretty obvious correlation. You'll also see that the guys at the top of the report are the 3-4 spot hitters (not that this is any revelation). The report helps to illustrate why RBIs have more to do with one's place in the order than they do with an extra level of skill. If you compare the percentages, you'll see that the guys at the top of the list aren't really doing anything better than the guys in the middle of the pack--the guys at the top just have more chances.