I had the same experience - replaced the battery with a new Optima sealed unit thru Interstate. I found that the reason the battery drained down was that the rear passenger dome light was burned out, and the switch was set to 'on'. This circuit also controls the lower courtesy lights in the bottom of the rear doors, which are so dim that I could not see them on with the dark interior, and they drained the battery. So do a current draw test off the new battery when it is installed looking for a draw that could kill the new battery as well! Good Luck.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E