I have slept in, at one time or another, a '69 Dodge van, '71 International TravelAll, '89 Isuzu Amigo and even a '69 Triumph Spitfire. (Jeez, I sound like a homeless person, when I write it all out like that!)
The best option was the full-size van where the back of it had been converted into a bed. It was quite comfortable and in the California weather, was almost never too cold, even up in the mountains. Out there, mosquitoes aren't much of a worry, so even when it was warm at night, I could crack the windows and be comfortable.
These days, hunting where it's too cold for such silliness, I sleep in my own bed, or in a travel trailer/RV/hotel bed. I would like to say I miss the "good ol' days", and there are parts I sincerely DO miss, but those impromptu sleeping arrangements are not one of them! I definitely hunt better after a good night's sleep, which I often did not get when sleeping in a vehicle. If I lived where it was warmer, I would consider one of those tents that attaches to your SUV or truck, increasing the "living space", as it were.