Master Electric Auxiliary Battery Issue

I have a Master too. A similar issue arose with mine, manifesting in comparable symptoms. It's rooted, more often than not, in the vehicle's power management system failing to adequately charge the auxiliary battery. Have you subjected the battery to a load test to ascertain its capacity under duress? What is the voltage when you measure with a multimeter?

Load test? No, I haven't done that. I don't have a tester. What voltage should I see? Where exactly did you measure the voltage?

Given that you lack the necessary equipment for a load test and are unfamiliar with the standard voltage parameters and specific locations for measurement, I would strongly advise you to consult a certified technician. You could also go to a workshop. Prematurely condemning the power management system would be inadvisable prior to thorough diagnostics. My problems looked exactly like yours though.

Okay, thanks! You were right; I took it in. Turns out, indeed, the auxiliary battery wasn't charging. They're replacing the battery. Total cost: 290€ Thanks for pointing me in the right direction!