Marilyn,
Like Fred stated, the battery isolator is used to separate the two battery banks from discharging each other, but it allows the engine alternator to recharge both battery banks. That is its only function. It is just a very large diode bank with the engine alternator output tied to the center terminal and the house battery bank tied to one end and the chassis battery bank tied to the other. Since the battery isolator prevents battery voltage from reaching the output terminal of the engine alternator, your coach came with a Duvac alternator with a sense wire and an ignition wire that enables it to properly recharge your batteries.
If my explanation does not make sense to you, and you need more help in understanding your coach wiring, just call me and I can walk you through your wiring issues. 713-254-4156 I am presently located in Arizona, (Mountain Standard Time Zone).
Gerald