a switching power supply that converts 400VDC to 12VDC is a trivial thing--there's one in the car already to charge the12VDC battery. Functionally the same thing as the external trickle chargers you're suggesting. Why doesn't tesla run the 12VDC components off of that and dispense with the 12V battery?
No engineer capable of doing any part of this would overlook that possibility unless there was a darned good reason. My guess is that the 12VDC loads fluctuate wildly and the worst case is several tens of amps. They chose to go with a small trickle charger and a bigger lead battery. That's probably a bunch cheaper than having a 12VDC power supply big enough to supply the worst case 12V load and is available off the shelf.
my guess is that the external trickle chargers won't help the problem at all....if they do, simply building a slightly bigger trickle charger into the car than the one that's already there would solve the problem trivially and telsla would have already done it. my guess is that they're working on whatever the loads are that give the 12V system a hard time but that they're proving challenging. do any of the folks that have put current monitors onto the 12VDC system have any thought what they may be?
--Snortybartfast
The architecture of the Model S has a safety feature involving the 12V battery. There are contactors inside the LiIon battery pack, which are normally open, to isolate the high voltage inside the pack. The contactors and closed by the 12V system when the car is operating. In the event first responders need to intervene to extract someone from the car, all they have to do is sever the 12V loop which is in the frunk to ensure those HV contactor stay open. That isolates the HV to inside the pack, making it safe for first responders to work around the car as long as they don't puncture the HV battery.
To feed the 12V from the DC-DC all the time would require the contactors stay closed; not a safe situation. If the contactors are allowed to open, then the architecture requires an external power source (i.e., the 12V battery) to close the contactors to get power to the drive drain when you want to drive the car.
The vampire drain is typically 1.2kWh per day. That's 1200 / 24 = 50W running continuously on average when the car is off. We don't know for sure what makes up that level of power, but here are some potential contributors...
3G/LTE radio (needed all the time for access to the mothership, unless on WiFi)
WiFi Radio (needed 100% of the time when in a hot spot)
Bluetooth radio (might be powered off when the car is off)
Key fob radio (needed 100% of the time when the car is off to detect approaching key fobs)
Center console CPU and memory subsystem (could do done much more economically by reducing the CPU frequency when the cars off)
Battery Management System (BMS). This is probably a big contributor to the vampire load.
In any event, 50W continuous on average is a HUGE vampire drain. A 12V charger from the shore power capable of supplying 50W (4.5A, say) would eliminate the deadly cycling of the 12V battery. That's a band-aid. The solution to the 12V battery longevity is to reduce the vampire drain by an order of magnitude at least.