More often than not the cargo door is already insulated to some degree. See if there is a soft foil looking material on the inside of the door. You could further insulate either the door or the inside of the panel with some styrofoam. (Lowes or Building Supply store, various thicknesses available.)
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.