There is a Battery Control Center, possibly made by Precision Circuits, that ties in the house and chassis batteries. Forest River installs it, not Mercedes-Benz. What do you need to know about it? There are some past threads here that have analyzed how it works.
It's a shame the dealer cannot help you with this, but I guess that is not surprising.
In my Ford based Sunseeker it is installed in the coach entry stairwell.
Here is a link to some typical devices for this purpose: