The crude but simple way to determine that is to remove the wire from the positive post on one or the other and see what stops working. Another way is to follow the positive wire to see which ultimately goes to the starter motor (and thus is the chassis battery), but that can be challenging on a Class C front end.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC