Probably to the holding tanks (black water and gray water if you have one). My '73 only has a black water tank, all the way at the back of the camper under the bathroom. There was a small, about 3", round duct from the main than ran thru the floor and into the side of the pan that holds the black water tank. The idea being that the heat would help keep the tank from freezing in winter. You probably have one near the fresh water tank as well, located just forward of the axels (I think that is where your fresh water tank is).