I apologize for the confusion, you are charging your house battery when the engine is running. The problem that is caused by being wired that way, is that it overrides the voltage monitoring functions of the ACR, and forces the ACR to combine the two systems. We really want the ACR to protect the van electrical system. If there was a heavy load on the house system , that causes the alternator not to be able to keep up, the voltage would drop and the ACR would disconnect to protect the van electrical system. The way sportsmobile wires it is wrong (IMO).
Just removing the red control wire from the sprinter circuit, would add that feature back to your ACR. As I said it would still be bidirectional. The following diagram shows as simple relay to essentially turn the unit into a unidirectional ACR.
Also you can use this relay with the remote switch, when the engine is off the ACR will default to the switch position (off - Auto - On)
You are correct you can't get SOC out of the zamp controller, solar controllers only know the current that they put out, nothing about the current being used.