a specially designed unit carried in the bed of a pickup truck
A unit loaded onto or affixed to the bed or chassis of a pickup truck.
Truck campers are campers loaded onto the bed of a pickup truck. The tailgate is removed and the truck camper is attached to the truck with tie-downs. This makes for a very versatile RV that can access back roads and remote areas other RVs can't get to.
In North America the term truck camper and its derived acronym, TC, are generally used to refer to any recreational vehicle or RV that may be carried in the bed of a pickup truck. In North America, this RV type is sometimes known as a slide-in.