I don't know much about an 83 240D but if your car uses the "dreaded" ACC servo (the box with rubber hoses attached at the bottom and a bunch of vacuum hoses on top) it is possible that it is stuck and will not park when the key is turned off. This causes a slow drain on the battery because the servo motor never shuts off. The only fix is to replace the servo. I found my problem by removing the fuse that powers the clock. Then I removed fuses until there was no drain on the battery. So, if you determine that you alternator is good, you might give this a try. I hope this helps.
Be kind, I am a first time poster to this forum.