New batteries have a quality issue these days

Personally I would remove the battery and have them test it on the test bench in the back of the auto parts store

To properly do the test requires 1 hour

Pm me to alert me of your results as I don;t normally monitor the V12 section

The V12 ,Iinline 6 and even the V8 versions of the X300 / X308 series is of about the same electrical layout

Parasitic drain is the term used when the car fails to go to sleep after he key is removed but continues to stay alive , but this is a large drain rate in your case

Quick possibility is the fan control module under the car fwd of the front left wheel is full of water as it hangs not as designed but water creeps in keeping power on the fan motors

The wires coming out of the module should point down

The part # for the X300 and X308 versions of this fan system fan control module are different and the X300 is white and the X308 is black

By removing the module and shacking you can hear the water if present

See page 81 :

http://www.jagrepair.com/images/Auto.../jagxj1996.pdf

Editing with pic