I run into this on a DAILY basis with my Toyota customers.
Usually wanting a dual battery system, which I promptly talk them out of, because it's easier to maintain one battery with our vehicles.
The first problem to overcome is the alternator voltage. Solved by using any one of the VS Booster devices found in the links in this thread. They are essentially diodes (which is why you get the Battery light on the dash if you install them the wrong direction) that reduce the voltage on the computers Voltage Sense wire coming from the alternator, tricking it into thinking it needs just a wee-bit more out of the Alternator.
The second problem to overcome is basic understanding of how batteries charge/refill capacity. The bottom line is TIME. If your battery rests at 12.9 and gets drained to 12.0-11.9, it is essentially dead in terms of capacity. Is it enough to voltage to still start your vehicle? Yes. But in terms of running the fridge or accessories for another night? NOPE, dead.
To bring that battery back from "dead" is going to take about 12-14 hours drive time if you're using the just alternator, even with the VS Booster. My opinion is that is what most of the people in this thread are experiencing; Just not enough drive time coupled with the low alternator output.
I don't run most of my accessories, including my fridge, when I'm not camping. So my daily draws are pretty low. I DO have the VS booster (mine is from Dirty Parts, but I like the HKB ones better; they fit in more vehicles because they consume only the space of a fuse), and I have no problem keeping my battery at 100% on a regular basis.
AGM batteries are awesome power horses, but if you're not running a ton of things like camp lights in the evening, or a fridge overnight, it's not worth the expense of just having it. On the flip side, even three or four of those Blue Sea USB ports drawing constant power are enough to put a good drain on a stock battery overnight, so pick your needs carefully.
When I'm camping, my 100W Renogy Solar Suitcase is my PRIMARY power source. It provides more amps into my battery than my fridge/camping accessories use all day, so the extra energy from the solar panel keeps the battery topped off all day long, because in reality, all I care about is being able to carry my fridge at 35°-40° through the night and still be able to start the truck in the morning.
I typically lose 5% to 15% capacity overnight depending on fridge setting/ambient temps. So by 10AM the next day, my panel has the battery topped off again.
AGM batteries have no issue with deep draws taking them from 13.0 volts to 12.0 volts, but what they require in between is a charge back up to 100% EVERY TIME. Doing this, having a battery that lasts for 8+ years is easy.
By the way, I saw some questions asking how to tell capacity: CTEK makes a great product called the Battery Sense (Not recommended for flooded cell batteries), it connects to your phone via Bluetooth, and will send notifications to your phone when the battery needs attention. It will show you, with a graph, how much capacity is left in your battery. They're about $50. You don't have to buy them from me, but please don't buy them on Amazon: There are a lot of sellers on there selling broken/old units. CTEK has made updates to the product, and many of the Amazon sellers are still selling old stock.
You can read about how CTEK themselves are dealing with this: BY VOIDING WARRANTIES.
