You may have to take sectins of the underbelly membrane down to find the entry source.
About all you can do is just start removing everything you can that is soiled..
I had field mice take up living in the air conditioner ducts in my trailer which is parked at the edge of our little woods. If you found activity around the furnace area then most likely they have free run of the floor heater ducts throughout the whole trailer.
Not an easy task to find the source and clean it up... Like stated above at some point you may have to seek professional help to get rid of it...
Roy Ken