I guess I'm an idiot. The heat pump is on another thermostat (the one in the bedroom). But, for the AUTO mode to work, don't the
furnace and the heat pump need the be operated by the same thermostat? On our last RV, the heat pump worked as long as it could
keep up with the temp, then change over to furnace when it couldn't or the outside air temp. went below a certain set point.
There are a fair number of complications for manufacturing to install and configure the equipment in a way that would take advantage of this thermostat feature. On some floor plans the furnace is controlled by the living room thermostat. On others by the bedroom thermostat. If you purchased 2 heat pumps, only one could be controlled without getting into a more complex zone installation. If the bedroom thermostat worked on AUTO but the living room thermostat didn't, you might not get much benefit except while sleeping.
If you just purchased one heat pump and one regular A/C unit, the heat pump would have to go in the area managed by the thermostat that controls the furnace, or the coach wiring paths would have to be modified.
I think the only way to make it all work together would be to go to a multi-zone configuration where one thermostat has multiple sensors around the coach. This would be harder and more expensive for Heartland, and I'm sure many people would find it difficult to operate.
From a manufacturing standpoint, having a single, simple configuration makes sense even though it doesn't take advantage of all available functions. More complexity slows down the line and adds to the possibility of getting something wrong.
I'd recommend turning on the furnace if you expect temps to go below 40 (F). If the heat pump and furnace are controlled by different thermostats, you could turn them both on and adjust the furnace thermostat a few degrees cooler than the heat pump thermostat. Then if temps drop below 40 and the heat pump isn't providing enough heat, the furnace will come on to fill in the gap.