The 3/4 ton and 1-ton truck frames have special reinforcement for handling the stress of a kingpin and towing a 5th wheel trailer. A 1/2 ton truck's frame is not likely to have this reinforcement and the kingpin mount needs to be 100% supported by the truck's frame and not the bed of the truck. Check the frames of the Ford, Ram, and GM 2500/3500 trucks and you will see that they are engineered to support the loads from a king pin. Check and see what your options are for mounting a kingpin to a 1/2 ton truck and whether it is going to be adequate.
The GVWR for the truck should be at least 10% greater than the combined weight of the truck and the tongue weight of the trailer. Towing capacity is not the issue so much as having enough truck weight to handle the tow load.
Some of the newer 1500 trucks are rated for towing over 17,000 lbs. so presumably their frames have been strengthened for mounting a pin for a 5th wheel trailer. I would want to verify this beforehand.
With a 5th wheel trailer in tow you lose the bed of the pickup for other uses. I would rather have a conventional travel trailer and have the bed available and be able to mount a roof rack on the truck for bicycles and boats and other gear. 5th wheel trailers are better suited for a sedentary lifestyle.