My travel trailer (not a FGRV) comes with the 3 holding tanks enclosed and insulated. The bottom area is where the heating ducts are...running around and between the fresh water tank, the black water tank and the grey water tank. When the furnace
is running everything is warmed in the sealed insulated area.
Any after market heater would be of little value if the area around the tanks were not insulated and sealed off from the cold. Many travel trailers are not suitable for use in the winter. Do your homework before you buy if winter camping is something you plan on doing.