I won't try to change anyones mind, but my experience has been the electric heater kept my rig warm enough to avoid issue when outside temperatures were in the teens. Furnace was set at 40ºF. But I don't think it came on. I never found it running when checking. Interior temp of RV was 52ºF.
Truth is there is no one size fits all answer. There is too much variation in the way our RV's are constructed.
A space eater alone maybe sufficient to avoid freezing for some, however running the furnace maybe required for others. Ambient temps are also a major factor.
One really needs to know their rig and make a smart decision. Overkill beats taking a chance everytime.