Yes, in really cold weather say <20', if you're using a "cube heater" with full hook-ups, I'd run the furnace
.....some to be sure and keep that space warm.
The path that the uninsulated furnace supply ducts takes to the forward bathroom, travels inline with the plumbing. The two rearward ducts that exit at the floor, in the twin models, also supply ambient heat to the space between the two shells and is purposefully left uninsulated. They, the forward thinking folks at Oliver, planned it that way.
I've used the in-house furnace, and cooked for three days in 20/40 degree weather and still had propane
left in the one bottle (20lb. tanks) and never touched the other bottle. Never had a notion of feeling cold.
However, using a "cube heater" while having full hook-ups is a pleasure. Although significantly quieter than most other furnaces, you don't have to listen to the unit kicking in.
These trailers are soooooo far superior to what most are used to with a fiberglass unit. The thought and research that the people at Oliver have taken to produce these units truly is over the top. It's all insulated down there.
I'm guessing that if you're only using a cube heater that not much heat would make it's way through the inner shell into the floor area below (hot air rises). That's the reason that the ductwork is not insulated, to keep that lower space between the shells relatively warm.
All being said ......... use
the trailer and enjoy the fact that, "You go where there are no hotels".