Roadtrek. Ok, if original set-up I wonder how you define b1-b2, is this from a battery switch? Or the fact there are two batterys?
My guess is there is a house,and chassis battery. If that's the case, the house Will never be wired to use the chassis battery.
If you have two house batteries, an amature separated the house batteries,not the factory. An inverter is considered a high load device and would not have been wired where you could only draw on one battery at a time by a professional.
To understand this study how loads effect discharge times. This can be studied on a website called the battery institute, if My memory serves. For me that study was in the early 2000's, maybe 2004 or so. So the website could have been lost,or the name may be different. That study was back when I was working on an electric/diesel hybrid boat.