Tom,
The battery isolator on your coach is not a switch, but it is just a very large diode that separates both battery banks, and prevents either battery bank from supplying battery voltage back to the alternator over the alternator output cable. Therefore, a sense wire is needed to supply battery voltage to the alternator regulator. Without this sense wire, the alternator does not know what the battery voltage is, and this usually results in overcharging the batteries.
There are 4 wires that are attached to your Duvac alternator. The large wires are for alternator output and ground. One of the small wires is ignition, and the other small wire is the sense wire. The sense wire has battery voltage on it at all times, but the ignition wire is only has voltage when the ignition is on. If the small wires are reversed, the alternator will cause a drain and overcharge the batteries, so be very careful that all wires are on the proper terminal. The sense wire usually runs from the chassis battery positive terminal or from the chassis battery post on the battery isolator to the sense terminal on the alternator.
Gerald