I threw my old twin size mattress in the back of my SB fleetside once. It was like 2-3 inches too long but i could close my tailgate if I curved the mattress up a little. We had an old tent with those fiberglass tent rods that I could curve and put in my bed stake holes, it looked like an old wagon, when I pulled a tarp down over the fiberglass rods, but it worked like a champ. The only problem now is that I cant sleep in a shortbed with the tailgate up(too tall).. my old longbed was great, i slept underneath the tonneau in that once, it was snug rolling over because of having broad shoulders, but it was possible. Plus, I slept outside underneath the stars and had sunshine wake me up, everyone else at the party was sleeping on the floor in the house, uncomfortably. I had a sleeping bag, pillow, etc :)
I never thought to look for a twinsize air mattress though, I wonder if that'd work for a stepside bed or not?
97 F250 7.3L CCSB
08 F350 6.4L CCLB
06 Ranger FX4 Lvl 2