When planning a trip up to Oregon last August and facing 100 degree+ heat, I needed a plan to take a box of our universally loved Palisade Colorado peaches to share with family and enjoy for the month we were to be away. I had noticed before that the storage under the driver side bed in our 2551 stayed relatively cool, and sometimes we would keep extra drink items there. I improvised a way to improve on the existing characteristics of the space by cutting pieces of 1" foil faced Styrofoam insulation and friction-fitting them into place in a double layer fashion, with a lid to match. The floor is already 2" insulation, but I added one layer there also to hold all of the sides in place. Even tho designed to be removed easily, it is still there, and will likely stay. Nothing fancy, but it works...
I use two one-liter water bottles and alternate them between freezer and ice-box daily. It keeps things cool enough for leafy greens and back-up beers even in 103 degree (outside) heat.
And the peaches survived to be loved by all!