Hi squishball, Your Aux battery voltage at 5.6 V indicates that it is beyond recovery, and will have to be replaced.
It is fortunate that you did not drive after the warnings, when this happens there have been reports of major noises from the transmission locking pall grinding into destruction, requiring major repair surgery and replacement parts.
It would be interesting if your dealer could very accurately measure the weights of your dead battery and the replacement battery, using a digital scale that measures to 3 decimal places i.e. 0.001Kg. This could indicate if the battery has boiled dry due to overcharging or if they are of identical weights, died from undercharging.
These AGM batteries have only a small volume of liquid electrolyte, as the plates are kept moist but not submerged or flooded in electrolyte.
I hope you are back on the road ASAP.
Cheers, Steve