Yes, the underbelly is only heated by the furnace. Most of that heat is radiated heat given off by the main furnace duct as it is not insulated, so heat radiated both up and down. On my '04 model there was also a 1" diameter hole in the bottom as an actual heat 'duct'. In my opinion, the 'duct' was worthless since air flow would be much less than underbelly air leaks thru holes in the frame, etc.
We camp year round with our volunteer work and rarely use the furnace, relying instead on 2 oil-filled electric heaters in the winter. Even w/o the furnace running, we have spent days with temps below freezing and never had problems with freezing lines in the underbelly, even though the exposed city water line froze along with the sewer drain. Also, the tanks are not likely to freeze due to the amount of liquid in the tank, but the fresh water pickup tube for the 12V pump is most likely the first item to freeze, especially since the fresh water tank is frequently located at the back of the trailer.
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK