Space heaters are great for warming your toes and the immediate area but do nothing for getting heat into the enclosed underbelly (unless you have done some mods to circulate air thru out).
The furnace is needed in cold temps to get warm air down into and around waste tanks/fresh water tank and valves (provided you have a duct from furnace into underbelly).
When temps drop below freezing we run the furnace on 65*F and use space heater in our area. Turn it down to 55*F at night.
We've been in -14*F temps for 2 weeks. 1st night water pump suction line froze up, thawed out the next day and then left inside access door open to water pump area, nothing froze after that......went thru a lot of propane but nothing froze