Freakin' Toyota engineers aren't the problem. Overthinking Overlanders are the problem!
The problem with the dual battery setup is this: You THINK your alternator can charge a second battery as well as it charges the first, but this is incorrect for a number of reasons. Your alternator is designed to run the vehicle systems while in operation, and the energy left over is intended to recover the starting battery from what it lost while starting your vehicle (very little loss actually), and keep that battery topped off, so that it's ready to go next time you start.
Adding an auxiliary bank of batteries, usually deep cycle variety, and then using them to depletion, causes a great problem. Because the only way to get them back to full charge is TIME. I don't care if you have two DC to DC chargers and a Generator. If you drain a 100 Amp Hour battery, you'd better not expect it to fully recharge in less than 10 hours. PERIOD.
So that means after your 3 day camping trip that drained your auxiliary battery, you'd better take the long way home if you want that battery charged back to full by just your alternator. AND/OR put an intelligent battery charger (CTEK MUS4.3 or equivalent) on the aux bank when you get home.
Why? Because while marine/AGM batteries are made for very deep draws, what they expect in between those draws is return to full charge. NOT doing so results in a dead battery YEARS before its expected replacement date.
Having done this for a living for long enough, I have customers who call up, upset their dual battery system isn't performing to their expectations. And the problem isn't the system I built, it's their expectations, and not having a clue how to maintain their system, and live within its limitations.
I've operated on a single battery installation for 6 years without issue or replacement. I use a solar panel when I'm camping; If I'm sitting still, and there is daylight, my panel is up and keeping my battery topped off all day long, because all I care about is running a few camp lights for a couple hours and carrying the fridge through the night. In the morning, I adjust the panel toward the sun, and within a couple hours, the loss from running the fridge is refilled, and the panel will keep it topped off all day again.
I don't run my fridge when I'm not in need of it. It doesn't sit in my driveway keeping a couple waters cooled all week long. Although it could if I set my solar panel up.
Can my single Group 31 battery run my fridge for a weekend without needing charged? Under ideal (not too hot out) conditions, YES, absolutely it can. But WHY would I want to abuse my battery like that if I don't have to? The Odyssey Group 31 is a 12 year battery if taken care of properly. But I have customers killing them within TWO YEARS because of abuse.
My single battery upgrade AND a solar suitcase saved me over $1000 compared to what my customers had installed for dual batteries. Proper mounting cages that don't fail under even the smallest of accidents are expensive (batteries are heavy, add inertia and they get heavier and become projectiles), DC to DC chargers or smart solenoids, cost of the second/third battery, and paying someone to wire it all up adds up quick.
If someone comes to me with the want of dual battery, they'll need to show why they need it; That their power consumption exceeds the capacity of the upgraded main battery, and that they're willing to go the distance to properly maintain the charge requirements of the auxiliary battery. I'll no longer install it, and then listen to people complain it didn't last as long as they thought it would.
Just because you can, doesn't mean you should.