Use stream of consciousness to answer the following. The fifth-wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. Some camper trailers use a fifth-wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle, and "fifth wheel" is therefore sometimes used as a synonym for such campers in North America. The coupling consists of a kingpin, a 2-or-3 ⁄-inch-diameter (50.8 or 88.9 mm) vertical steel pin protruding from the bottom of the front of the semi-trailer, and a horseshoe-shaped coupling device called a fifth wheel on the rear of the towing vehicle.

Why is it called a 5th wheel trailer?
The relevant sentence in the passage is: Some camper trailers use a fifth-wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle, and "fifth wheel" is therefore sometimes used as a synonym for such campers in North America. Final answer: Some camper trailers use a fifth - wheel configuration , requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle.