Just a couple of thoughts on this one. The isolator is O.K. because the voltage output of the alternator is being seen on the output side of the isolator as measured elsewhere, dash etc. I'll asume that both diode sets in the isolator are good and that both house and chassis batteries see charging voltage levels of 13+ volts. The bench test of the alternator indicated that the regulator was working because the voltage was held at 13.5v while the output current was tested at rated output I will assume, approaching 165amps or so. There are several smaller leads typically on these alternators, like the Leece Neville configuration, that are used to remote sense the voltage levels and then regulate the field current to adjust the alternator output. Your voltage levels are very high indicating that the field current is turned fully on most likely due to a false low voltage condition being sensed on these leads. If the regulator was O.K. and the sensing voltage was right the output could never climb to 18 to 20V because the field current would be adjusted by the regulator to maintain around 14v under various current loads. The root cause can be an open circuit breaker, a bad diode in series, a broken wire, or an open ground wire as part of the sensing circuit. I would measure each of the smaller external wire voltages with the ignition on and off just to see that some voltage is present. If you have a friend with a similar coach they may be able to provide useful voltage information or an alternator shop may be able to give you the proper expected voltage range at these terminals. Hope this helps...