An Austrian company called GentleTent produces inflatable tents specifically for vehicle-based camping. When packed, their GT Pickup tent is a diminutive, 7-inch-tall package that fits neatly in the bed or on your roof bars:
Inflated, it expands into a massive dual-chamber tent with seven feet of standing room in the main area over the bed. The sleeping chamber perched on the roof is large enough to fit a full-size mattress.
If you're camping in a windy region and would rather sleep on the ground, they offer this GT Box "Add-a-Room," rated to handle 70+ mph winds. It packs down to fit in a 12" x 24" bag, and expands into a roughly 8' x 8' room:
To block the draft coming in from under the vehicle, they make this GT Seal Tube: