how to insulate a travel trailer?

Styrofoam? 2nd hand cold room panels? A layer of scrunched up newspaper between ply sheets? Bubble wrap on any exposed windows. Bricks? Carpet? The trick would be not further lowering the inside temp when you're accessing the stores. Or have a bucket or something inside you could chuck a hot rock or brick into to offset the cold air getting in. And maybe put a thermometer inside where you can see the temp from the outside.