The open area under the sink is the air intake. There is a removable panel above the opening, at least on my trailer. Before you plug it up, get the installation instructions and check for minimum air flow requirements. If you restrict the air flow below the minimums, the furnace could overheat and create a fire hazard.
Are you sure a duct didn’t come off? It makes little sense to have an output and the inlet in the same location.
“You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO WBCCI 1322
2002 Classic 30 Slideout -S/OS #004
2013 Dodge 2500 Laramie 4x4 Megacab Cummins