I just spent two weeks sleeping in the back of my truck with a camper shell. The best addition by far was a bed rug. That combined with a thermarest neo-air and I slept as well as I would have at home. It is hard to overstate how nice a bed rug is. It makes a soft, flat deck that is warm and easy on your knees. Gear doesn’t slide around and it keeps dust from coming up through the bed holes for drainage (and gooseneck holes).
