How Do I Identify Which System Is Draining The Battery Over A 48 Hour Period ?

depends the meter...
A better way would be to use a high wattage, low Ohms resistor...
I've got a switchable 0.2 to 8 Ohm load board.....
Can dissipate 500W with a fan.... more for short durations.
Put that in series and measure the volts drop across it..

 

Parasitic currents are a nightmare.
We've an old E240 Merc and it kept draining the battery intermittently.
Measured the draw... near zero.
New battery... did the same.

Eventually found that it would switch on one front side light when it wants to.....
Because of the way it is parked we never noticed.
Something to do with the big lightswitch assembly..... the car is worth about £250 scrap and is used purely for a backup, so we just disconnect the battery.

 

https://www.dropbox.com/s/pg0q50z2c17w5s1/DUCATI ELECTRIAL DIAG TRAINING.pdf?dl=0

 

Definitely a smart option .... you know your electronics well

I know it well ..... had a mid-seventies Merc saloon that had options to use left parking lights ( front and rear ) , or the same for the right ..... depending on which side of the road you parked .....
and I often failed to notice that they were left on .