Battery Separator issue?

Hi all, I’ve been noticing that at night when my truck(2021 Vnl 860) is parked and my electric APU is running it’s running down my starting (truck) batteries quickly. Like down to 12.2 just sitting overnight. I replaced both the starting and APU (aux) batteries within the last couple months, decent lead acid up front and a good AGM in the back. I’m thinking the battery separator must be stuck connecting the two battery banks. Does anyone know how to test the switch? It’s mounted right behind the starting batteries. This is really frustrating me. And looking online I can’t seem to find stock on any of these separators. Wondering if Volvo stocks them