We used to use camper shells on our hunting pickups. I don't think anyone actually insulated them although a few people used small propane heaters when it was really cold. I slept all over the Eastern half of the country in my S-10 while traveling to archery tournaments as well. I had a 6 inch foam mattress and even slept in the S-10 with two people at times although that in sleeping bags with our heads on opposite ends.
One friend had a bed down one side with storage in the front on his full size pickup with shell that was slightly taller than the cab. I've also slept in a half dome tent in the back of a pickup. If your bed is short, that might be a good option as well.
