George, with MendoBob's proposed system he wouldn't have any physical switches. The factory aux battery would just do its thing in the way Mercedes designed it to. The house battery would do its thing based on the SmartPass cutting in and out as it senses an alternator-provided voltage rather than a starter-battery provided voltage.
The issue is, will it actually work the way it's intended to, especially with the solar charge element of the system.
If he trusts Mercedes' engineers and only runs light loads from the aux battery, he should be fine with just the one monitor on the house battery. Adding a monitor to the aux battery is obviously also worthwhile to know what SOC is, but there's not much that he can do to change that SOC without running the engine.
I'm sitting on the sidelines hoping that MendoBob goes ahead with this system design because it's a really interesting way to combine the aux battery and a third "house" battery. I still believe that
the aux battery isn't an ideal option for a conversion van, but in MendoBob's situation and with the unique usage pattern he describes, it could be a viable solution.
On the other hand, I agree that it is adding a lot of complexity where a replacement/alternative battery for the aux battery might simplify things.