In other such systems I've seen, the shunt should be between the batteries and chassis ground. I can't tell if that is the case from your diagram - is that what the "RV" in the picture means? If the battery bank has any other pathway to ground, the shunt will show nothing or incorrect amps.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL