No takers so far

Well, now you have two batteries! One for use, and one to carry around in your trunk (fully charged) to swap in and never be stranded again.
Once again, problem kinda solved! Lol

 

If you have a "highly variable parasitic drain" then a battery isn't going to solve the problem.....INCLUDING the 266VDC Li-Ion battery pack that you're trying to tap into!

You NEED to find the source of your current leak.

Usually they fall into some predictable categories, listed in order of probability:

1. Accessory inspired wiring mod
2. Accident - improperly repaired.
3. Manufacturing defect.
4. Manufacturer design flaw.

G5s are three years old - and I'm not seeing many people complaining about having to jump dead batteries in these cars over and over again. They DO have under-sized batteries because of corporate greed - but they ARE generally sufficient to stay in service for 3-5 years.
You've 'likely' already engaged your local Toyota dealer about this since the car is only 2 years old - and even Toyota's sad warranty will cover a wiring issue in a vehicle less than two years old - unless you've already boltered through the mileage limit...

You can find some kid to manufacture a DC-DC circuit to trickle charge your AUX (13.5VDC) battery from your Traction (266vdc)...easy peasy!
HOWEVER (comma!!!!)
Such a modification would immediately and justifiably void your vehicle's warranty, AND for a pretty good reason!!!!!
You've PROVEN yourself that if you have a "highly variable parasitic drain" that draining TWO batteries with the "highly variable parasitic drain" is MUCH MORE than twice as dumb as ONLY draining the 13.5VDC battery.

Advice:
1. Fix your car.
2. Get a jump pack, and move on with your life.

 

lots of people have had 12 volt problems with the g5, and there are many threads here