As Richard S mentioned, our National Dolphin has at lease one, maybe 2, flexible ducts going thru the floor from the furnace and into the enclosed space where the tanks and other plumbing is located. We use electric heating as much as we can but one needs to let the gas furnace run if heat is needed for the tanks in freezing weather.
Ellen & Greg
08 National Dolphin DL-35Ci, RoadMaster Rear Sway Bar, Rear Trac Bar, Koni Shocks Front,
Workhorse W22, Kelderman Air Ride, Towing 2016 Smart Car