JTPSL500: My 1990 500SL has did the same thing a couple of times in the last two years and it seemed to be caused by the power antenna. When my battery is not quite fully charged and I shut the key off the power antenna does not rertract all the way and it will kill the battery. Try unplugging the power antenna in the truck and see if it still kills the battery. It's a long shot but easy to ckeck. Good Luck!