Dave,
Looks like you and Gerald are hot on the trail of the voltage regulating problem. By putting the resistance heater load on the inverter you are able to drop the output voltage back below the alarm spec of the Xantrex, but you are doing that by putting a severe load on the inverter and alternator. The alternator is trying to hold the output at 16v (for your case) and the best it can output is 13 to 14V under load.
Whatever the amps are at 120V, multiply that value by a factor of 11 and you will be close to the DC amps that the alternator has to supply for the inverter load alone. Then you have to add the 12V coach load, that is being used over the road, to power all the other 12V needs. 100amps, which might be a sustained safe load for a 150 amp alternator, gets consumed fairly quickly.
You are between a rock and a hard place with possible over voltage DC damage to some 12v equipment vs a possible burned out alternator scenario. I would avoid running the coach until this is fixed if at all possible.
Later Ed