Some people only buy an RV trailer for vacations or occasional trips from location to location; in those cases, a little bumper-hitch travel trailer will usually suffice. But for modern-day nomads, who call the highways "home," and for whom ceaseless motion is a part of life, RV trailer manufacturers offer a number of very serious tractor-trailer couplings. The fifth-wheel hitch is one of the most popular, but it isn't the only option out there for moving the biggest and best RV trailers.
The "fifth wheel" got its name from the pivot originally used to join the two halves of articulated, four-wheeled, horse-drawn carriages. Originally, the fifth-wheel literally started out as a wheel laid flat, and mounted to one half of the carriage on axle pins sticking out of the edges of the wheel. These pins allowed the wheel to pivot up and down; the second half of the carriage contained the "axle" that passed through the senter bearing hole in the pivoting wheel. From there, the "wheel" evolved into a horse-shoe shape, with a slot in the back that allowed the axle -- now known as the "kingpin" -- to slide in from the back rather than drop down from the top.
The slot in the back of the wheel allowed tractor drivers to simply back up to trailers to connect, but it also allowed the trailer to simply fall out. To fill in the slot, the fifth-wheel contains a spring-loaded coupler arm that swings in behind the kingpin. As the driver back up to the trailer, the kingpin slides into the slot until it hits the front of the central pivot hole. At the front of that hole is the end of a ratchet arm that holds the coupler; when the kingpin hits the arm, it releases the spring-loaded coupler, which swings into place behind the pin and locks into a recess in the other side of the slot. To release the coupler, the driver sets the brake on the trailer to hold it, backs into it slightly to release pressure on the catch arm, and pulls a release handle on the fifth-wheel to pull the coupler back to its starting position.
A big Class V bumper hitch can pull upward of 17,999 pounds, and can hold 1,200 to 1,500 pounds of tongue weight -- that is, weight pressing directly down on the hitch. A fifth-wheel hitch like those used to pull RV trailers can hold upward of 24,000. That's good if you have a massive trailer, but probably more important is that the fifth-wheel's only real tongue weight limitation is what your tow vehicle's bed payload capacity. That fact allows you to tow a trailer with much more forward weight, including a bed over the hitch. That has a ripple effect through the entire towing and living experience; putting the bed over the tow vehicle makes the trailer shorter overall, and shifting the weight forward plays an important role in safety.
A rear-heavy trailer can easily develop a pendulum effect during quick lane changes, or even normal, small corrections while driving. The swinging trailer will force you to make larger and larger corrections, creating a dangerous "snake" that may cause you to lose control. The fifth-wheel also makes parking much easier; with a shorter distance between the hitch and trailer axle, and a shorter distance between the hitch and steering tires, the fifth-wheel trailer responds much more quickly to driver inputs while backing. This can allow you to easily maneuver the truck-trailer combo into the kind of tight and oddly angled parking spaces you'll often find at RV parks, rest stops and truck stops. This principle applies while moving forward, to, since those shorter distance mean that your trailer off-tracks far less while turning; that means you can take the trailer around city corners without running over every fire hydrant in town.
Fifth-Wheel vs. Gooseneck
A Gooseneck trailer is identical to a fifth-wheel in principle, but upside-down. With a gooseneck, the kingpin is in the back of the tow vehicle, and the fifth-wheel coupling is on the trailer. The gooseneck has two major advantages. First, typical gooseneck systems can pull upward of 30,000 pounds, because the kingpin plate mounts more securely to the tow vehicle than the fifth-wheel mechanism. It also mounts flush or almost flush with the bed; with the mounting brackets under it, a fifth-wheel comes right up to the top of the bed, and takes up a lot of space in the bed. Gooseneck kingpins can be dropped down below the bed, flipped over or removed when not in use, allowing you to use your truck as a truck when you're not towing. Gooseneck assemblies are price-competitive with fifth-wheels, but installation is more involved, and trailer selection might not be what you're looking for.