In some cases, people use a rear support system. This involves mounting a rack on the back of the 5th wheel and attaching the kayak to it. A few companies make systems specifically for kayaks that attach to most 5th Wheels.
Others choose to use their front bumper assembly to help support the rack and the kayak. This is often more preferable, as it makes transporting the kayak easier. Some 5th Wheel manufacturers offer modifications that make this easier, as do some independent retailers.