I would imagine the furnace works more efficiently in a 2100 than it does in larger PCs.
I wonder if Phoenix uses furnaces with different BTU ratings pending the square footage of the model, maybe even with consideration to slide outs and thermal windows. The same furnace used in a 2100 no-slide versus a 2910 with triple-slide is a lot of variation.
I am pleased with the furnace in our 3100. There are three vents... at the kitchen sink, at the passenger side bed and the bathroom (all at floor level of course). But, we are in south, coastal Texas where the temps rarely go below low 40's at night. On occasion, we have had to run the furnace to knock off the chill and the temperature in the bedroom would reach low 80's. I'm convinced that the furnace would do a good job in colder areas. Unfortunately, we are having to leave Texas early and head back to Tennessee later this month. The furnace will be put to test.
We do have a small electric heater in the bathroom for taking showers and a larger one in the main living area. They both do a great job.
Oh, I failed to mention that most chilly mornings, the heat pump does a great job in knocking off the chill and then use the portable heater until it warms up outside.
Oh, again...Ron, I have no idea what size furnace our 3100 has. I could look at the manual, but that would cause me to rouse my wife from the driver side bed which would not be pleasant!
I'm thinking that even in temps in the low 30's, it would "run you out". Again, I will soon find out.