It's strange because the 1500W Auxiliary Power Module should be providing 14V to the automotive systems continuously while in use. If BustosMan exceeds the ~110A on the 12V DC system and the AGM voltage sags below the voltage needed to keep the contactors energized (or the BCM/ECU calls it quits before the contactors move for safety) then the car could go into a "not ready to drive" state.
Was there anything that could be drinking 12V power that you know of? Aftermarket stuff?
I had a scary experience a while ago where an anomalous dead short persisted after starting a Spark from service mode at about 50% SoC. The terminals were at least 80ºC from current likely exceeding 100A. The car failed to start and everything including the instrument cluster was dim. I made a post about it.
I like MrDRMorgan's suggestion. If the AGM can be reconditioned or replaced if its capacity has degraded too much, that should ensure you don't have any problems in the future. It's always good to have a voltage tester around to periodically see what voltage it's sitting at too.
The Spark should be maintaining the AGM for at least a month, maybe two after you last drove or plugged it in. After that point the AGM will begin self-discharge.
If you have a dash cam and leave that running in parking mode and sufficiently drain the AGM, then the APM might not charge the AGM back up in time if your trips are generally pretty short. AGM charges faster than flooded lead-acid safely up to around 0.2C compared to the older technology at 1/20C, So it's a question of if your net energy usage is negative, where you're draining more energy than you're typically putting back into the AGM pack.