Remove the 3 larger wires from the isolator. Be careful. With the engine running the wire with 15 + - will be the alternator. That goes on the center. One of the other wires is the coach battery and should have the same voltage as the coach battery with shore power unplugged. The last wire will go to the chassis battery and should have that voltage on it. If you have 0 volts on either wire you have a broken circuit to that battery. If the voltages are correct, they go on the outer terminals of the isolator. To check the isolator, voltage is allowed to go from the center to both outer terminals but not back. An ohm meter will clear this up quickly.
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