Unless it can be damaged or leak from freezing, we pretty much leave it in. Linens, jackets, pillows, rolls of paper towel, toilet paper, hiking and bike route guides, maps, what ever, all stays in.
Food comes out, all batteries get removed from devices because they can leak and ruin the device (even if you take them in the house), any bottled water or other liquids that could freeze and explode. The trailer battery gets stored on a shelf in the basement.
As far as mice, it depends. Generally if there is no food for them they will stay out. However if there are bushes, trees, or weeds around your trailer they may find your trailer to be a nice home. We don't have any of that around our trailer and it's stored on a gravel pad next to our garage. 23 years of storing various trailers there and no mice, ever. They love our shed and lawn/snow equipment though because the shed is a few feet from the tall grass along a creek behind our back yard.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE