I have pluged all visible holes/locations for mouse entry under my Classic with bronze wool (available at marine stores) saturated with silcone adhesive to form sort of a wool paste. Mice won't chew through the wool and the adhesive seals the hole so they won't push the wool aside. As a precaution, I place moth balls in places such as under the wardrobe where the water pump is, under the sofa near the battery boxes, under the bed and under the bathroom vanity near the water heater. Just playing it safe.
With all my precautions, I was suprised to find my power cable compartment filed with mice excrement - soaking wet and stinky this spring. It took multiple cleanings with various products to cleanse the compartment. I guess I need to place moth balls enmasse in that compartment when I winterize this fall.
2004 Classic 25 - The Silver Sausage
2008 Silverado LTZ CC 2500 4x4 Duramax - Brutus