Furnace Ductwork Keystone Outback 323BH

I'm searching for any picture or diagram that illustrates how Keystone achieves "heated underbelly" for a 2013 Outback 323BH. For example, "Is there a port in the duct that runs under the floor?" I see no direct connection, through the trailer floor under the furnace (Suburban SF-30FQ). Only connections to flexible duct work routed through the wall for one register and two routed under the floor (apparently) other outlet registers. Any insight will be appreciated.