It's even stranger in a Sprinter chassis.
The alternator will charge the coach batteries; but the converter won't charge the chassis battery.
An agreement between the coach's manufacturer and Mercedes prevents any electrical connection from outside the supplied chassis going into the electrical system that is installed by Mercedes. All chassis manufactures have some sort of restrictions on what the house manufacture can do or modify. For instance specifies the max frontal area of the Axis/Vega. That is why the front cap is smaller than the body width.
Jim & Roy Davis
2016 Hurricane 31S
1961 Rampside in tow