I have had the opportunity (?) to try both of the scenarios you are considering. While I ordered 2 heat pumps on my new BigHorn, the factory eliminated the heat pump in the living area and included one for the bedroom. While I was waiting for the new heat pump unit to be delivered and installed I spent 2 weeks in the Horn with mild evening temperatures, lows of 36ish to 45ish. While the heat pump does not respond as quickly as the furnace to heat the coach, it does an excellent job of maintaining the temperature. Heat pumps are also great for keeping the coach cozy while you are away without using propane, presumably on the parks electrical nickle. The heat pump will heat the bedroom until you can't take how warm it is in there. We placed a floor fan--directed towards the living area--and the one unit did a pretty fair job (combined with the living room fireplace) of keeping the entire rig warm. On several evenings we selected a Low manual fan speed, away from Auto, and we both slept quite well with the heat pump keeping the room at our set point of 62. If you don't select a manual speed for the fan the unit cycles and disturbs sleeping. I would estimate the fan noise on the same level as a thru-wall mounted air conditioner in a hotel room.
Once my second heat pump unit was exchanged/installed, I was curious as to how efficient it was working. So, I zapped it with my super-duper, laser-guided, Binford model 702 thermal gun (argh, argh), aimed precisely at the ceiling output vents in the living room. With the outside temperature at around 45 degrees, my blue-background display indicated 114 deg. So, I decided the new unit is working. Operationally, you cannot operate both the heat pump and the furnace at the same time.
For low outside temperatures: Between 40 degrees and 32 degrees the heat pump with alternate between operational and defrost modes. At 32 degrees and below the unit will transfer to furnace operation for the living room unit. If you care, you can view the operational activity on the Dometic thermostat display via the icons. Also, the unit will automatically switch from Heat Pump (HP) mode to air conditioning for those days when you have chilly mornings and warm afternoons.
Now, each of us will decide if all this is worth the expense. I find that the relatively small expenses are forgotten in the long run. But, don't expect to have any of them added to the value at trade-in time.
