Do I need a stinger to totally isolate the auxillary battery from the main battery or does the TrueAM do that for me?
Like if I wanted to listen to the tunes and run the aux battery down while the machine is off and still have the main battery to start the machine.
Couple of additional things to consider. Lead acid batteries last longest if not discharged below 80%, and going down to 50% shortens their life even more. As soon as you start to use a lead acid, the voltage starts a downhill drop. Going below 50% charge, as mentioned, is tough on the battery (especially so if not charged up soon thereafter).
Lithium batteries, as shown in the chart below, maintain their voltage for a lot longer. Unlike lead acid, they don't suffer degradation when discharged almost all the way down, so practically speaking, they have a lot more energy density. (Note that the battery in the chart is Lithium-ion. They are well know for being problematic, with a penchant for toasting marshmallows if everything isn't just right - just ask Chevy Volt owners and, on occasion, Tesla. Much safer, but a little less energy, it Lithium Ferrous Phosphate (LiFePO4). True makes a version of their battery isolator to work with a LiFePO4 aux battery. They are also lighter than lead-acid, oh, and significantly more expensive. The pay back in way more charge/discharge cycles.