I assume the dealer checked the alternator charging functionality.
I am not an expert but you should be able to measure across fusbox what is drawing current with vehicle parked up with a decent digital multi meter.
Failing that adopt a trial methodology (which obviously takes time)record and take out half the fuses and observe results. If battery OK then you know the circuits served by the fuses left in place are OK BUT one of the circuits served by the fuses you removed MAY be the culprit etc etc.
Good luck and keep us posted.
230 TE (W124) 1989 with 153,000 miles on the clock - hoping for at least another 100K