I asked around on a few RV forums about cleaning heating ducts they recommend removing the furnace to access the duct work. Work from both ends of the ducts with a garden hose connected to a shop vac.
Depending how your trailer ducts are setup you could pour 10% bleach from the inside catch in a pail with the furnace removed? Only the deer mouse carry's the virus catch a few to determine the species.
The construction of these units are truly despicable, in the trailer I had mice the penetrations for the water and electrical had a 2" hole for a 3/4" hose leaving a hole you could throw a cat through. On my latest the 12 volt battery connection is reversed my black wire is positive???
A good article for furnace cleaning
http://www.rvdoctor.com/2002/02/rv-h...intenance.html