That pretty much sums it up. For as long as you think he would be be okay to play outside in the cold is about how long you can go in a trailer. You can go typically go a bit longer because they are protected from the elements to some extent. But they are not moving around much. You can pack blankets and pillow around them to help insulate. It helps to put a blanket or pillow under their feet because those floors are cold (although he probably won't be reaching the floor for awhile.