Check out ThermaRest's Neo Air Self Inflating Sleeping Pad
It is super light and can deflate down to the size of roughly a pint glass.Which will fit in the one of the compartments surrounding the spare tire. I ordered the large one, I'm 6'4. It fit in the Jeep if I angled it just the right way, but i kept sliding down toward the rear hatch due to the uneven parking spot at the campsite. Even though temps were hovering around freezing (this past winter in the Mojave Desert) I found that the inside got super stuffy with me and the dog in the rear and my buddy sleeping in the passenger seat. It was better than freezing outside though.