I have a Tiffin Allegro Bus with the heat pumps. When you see the dealer again, see if the heat pumps are "self-defrosting"; some are and some aren't. Ours are and we are able to run ours even lower than 40 degrees. As Ned says, the heat pumps work VERY well. I don't know if your Phaeton is like our Bus, but when the temp gets down to the freezing level in the bays, our LP furnace turns on to heat the area and keep the tanks from freezing. Ours is a 2003 so I don't know if they offer that in the newer coaches.
Most furnaces are LP only.
Enjoy your Phaeton.