Howdy Anders and welcome to the forum.
Do you hear any contactors clacking when you turn the key to try to start? What about when you plug in to chargge the pack--do you hear contactors, and fans and pump motors whirring?
Do you know the condition of the 12-volt aux battery--is it the original or has it been changed out recently? a weak, old or worn out aux battery can cause issues such as this. If it is the original, it is likely worn out and should be replaced, even if it shows 12.5 volts with a multimeter.