Mouse nest in heat duct

I had the same situation a few years ago with our KZ Spree so having been there, done that I'd suggest you won't find the problem nest(s) at the furnace outlet but rather within ductwork inside the trailer. In my case I was lucky in that the ductwork was flexible foil that ran above floor throughout various cabinets and after discovering that simply cleaning the ductwork wasn't going to work at all I replaced them, a relatively easy task since that ductwork was readily accessible. However, if you have in-floor ducts with the ductwork running along the underside of the trailer floor which is enlcosed with Coroplast then I can guarantee you have a real job ahead of you. :E