I put the Adventure Wagon auxiliary battery kit in my van with AGM GC2 batteries under the hood back in 2018. If I were doing the same thing today I would be considering LiFePo4 battery technology and putting it inside the van instead. The temperature extremes under the hood can be challenging for batteries in both directions.
I found this past June the northwest's elevated temperatures (90°F to 100°F) put my battery temperatures up to at least 120°F, which inhibited charging levels (temperature-compensating charger), which inhibited my refrigerator's "super cool" function. (Isotherm "Smart Energy Controller" runs at a higher level of cooling when it senses the charging voltage, but the temperature compensation reduced the charging voltage to the point that the SEC didn't do the "super cool" thing. That resulted in my refrigerator *not* being super cooled when I stopped for the evening. Since the van warmed up considerably after I stopped, the refrigerator ran longer and used up more battery. In other words, the system stopped working properly just when I needed it most. Conversely in the winter time it is possible for the battery temps to get too cold; even AGMs don't charge as well below 32°F (0°C).
I think the aux batteries under the hood is an ok solution for occasional use, but has its drawbacks when you encounter environmental extremes.
WIth respect to AGM versus LiFePo4, I think the battery technology has advanced quite a bit over the last 3 years, and I'm considering replacing my AGMs with LiFePo4. The impetus for replacing so soon is that one of my AGM batteries (2 six-volts in series) has a failed internal cell, which most likely came about while my van was sitting during the first COVID year. I think LiFePo4 is less likely to suffer the same kind of failure. LiFePo4s accept charge faster, don't need to be kept full to the brim all the time, and have a higher usable amount of energy storage. Their main drawback is that you can't charge them below 32°F, but most of them come with an internal BMS that keeps that from happening. Although they can be stored outside (especially if you get ones with an internal heater) they are considered safe for use inside your living space because they don't outgas, and so its easier to keep them from environmental extremes of temperature.
They cost a bit more than lead-acid batteries on a raw amp-hour basis, but factoring in their higher usable energy storage, their increased cycle lifetime (charge/discharge), their faster charging rate, and their lack of the same "keep it as full as possible all the time" requirements, the cost difference may not be that big IF you depend upon your auxiliary power source being useful.