I put a worn out Cabellas Self Inflating queen size mattress back there for the dogs. Does a great job of covering the space between the back of the front seats and the front of the folded down rear sets. Also provides some protection for the rear arm rest. A definite keeper in my book. It does end up folding in the cargo area over the wheel wells.
Short answer is yes. The distance between the two wheel wells is 43" and I'm seeing that an average twin mattress is 39". I was hoping to fit something larger than that as we're driving cross country this summer. I'm thinking 2 Thermarest Dreamtimes at 25" a piece will be ok, but will be lifted on the sides.