Hi Keith,
The round vents in the bath and bedroom wall are only for the propane bed/bath furnace which is controlled by its own thermostat. You might be having a draft coming in through the fresh air supply for the furnace.
The heat pump will probably not work at 36 degrees - depends on the humidity we have discovered. Sometimes we will have marginal operation at 36 degrees, sometimes it won't work at 42 degrees period. It sounds like it is too cold for the heat pump and the controller is automatically starting the main propane furnace.
Another reason the propane furnace might have started is if the difference between set temperature and room temperature is more than five degrees.
Welcome to the forum! Nice coach you have there...