We have loads of mice in a culvert behind our lot. They infest our lawn and snow removal equipment in our shed. They get in our attached garage and have even got in the wall on that side of our house. Yet, never in 22+ years of storing trailers next to our house have we had mice in them. Don't give them a reason to enter it.
Keep all food either sealed in plastic containers, or remove it all together. Keep the area around and under the trailer free of weeds and other hiding places.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE