Although not too common in the US, most dual battery systems we find installed out on the trails and expeditions we are on, are usually neatly tucked under the hood of the car, or inside one of the interior panels in the back, to give a factory look, and be out of the way. Although very clean and factory looking, these systems only serve their purpose well, if that vehicle is only used for expeditions or overlanding either standing in between, or constantly on expedition. If they remain in a vehicle without being used throughout the year, they either wear the battery out if it stays connected, or puts extra strain and wear on the battery if disconnected, just from the shaking and forces on it. What works best is to disconnect and remove the battery when not used, however that is quite a hassle and leaves loose cables lying around in the engine bay and a unused battery monitor in the cab that could suffer accidental damage from bumping it or putting something on top of it, even possibly having kids stand on it (parents would understand).
Our solutions for this, and our favorite options for a dual battery set-up is a portable auxiliary battery box, which can be easily moved between vehicles, and even be used outside of the vehicle in the house during power outages or on camp sites. What makes it even more versatile, is like in the case with our 24V Land Cruiser, if it is fitted with a DC to DC charger that can also handle 24V input, then you can use it to not only power extra accessories in your 12V vehicle, but also to power 12V accessories in your 24V vehicle (like many Japan import four wheel drives are).
Pre-wired for you, including a charger and most of the port you will require for your overland accessories, this box is ready to go as soon as you throw a battery in it.
Must more affordable than most of the other chargers on the market, and the major benefit of this one being that it accepts 12V as well as 24V which is now more common on imported Land Cruisers, also used in solar panel systems and marine applications so you portable auxiliary battery box can be moved between all you toys with this charger.