It takes a churnin' burnin' hunk of truck to pull a fifth wheel portable mansion. But then, there's the lightweight rig that's happy with an urban warrior pickup. It depends on your wants. In any case, an 8-foot bed is what you need.
The largest fifth wheels weigh 14,000 to 16,000 pounds and you won't be towing them -- or stopping them -- empty. They typically are more than 32 feet long and have several slide-outs (sections that slide out to make certain sections wider when parked). Some have granite countertops and tile floors in the kitchen. All that rock adds weight. You're probably looking at three axles, sometimes four, and six wheels at least. Get the biggest truck you can find with the largest diesel engine. Do your homework on the camper discussion groups. People generally learn from their mistakes when they get stuck on the road with a blown engine or transmission. The more weight you're pulling, the more weight, power, suspension stiffness and brakes you need. In this class, you need at least a 2-ton truck with a dually rear-end. Many people with this large of a tow go for a mini-tractor or even a full-size 18-wheel tractor. If you're thinking 1-ton, that will work if you're a snowbird who only pulls it twice a season. Don't use a 1-ton full-time. Experience shows that it will wear out the truck, especially the transmission.
Although it is still a good idea to go with a diesel in this weight class, a 1-ton dually is ideal and a 3/4-ton will get the job done in a pinch. If your expected terrain is flat and your travel infrequent, the smaller truck will be fine. If you're pulling the rig through the Rockies and you're full-timing with all your belongings, go for the big package. It is important to keep crosswinds in mind, too. More weight means more stability. A wider stance like a dually helps with high winds and cornering.
Something 8,000 to 12,000 pounds empty will weigh about a ton more loaded. A 3/4-ton truck with two wheels in the back and a gas engine can do the job on the lighter end of the scale. On the upper end, you might want to opt for a diesel engine to get the torque you need for those long hill climbs. If you happen to have a 1-ton dually to pull this weight class of fifth wheel, you won't even feel that it is back there.
At the high end of this category, you can get away with a gas engine 1/2-ton truck. At the low end, a compact truck with a six-cylinder engine will do the job. There are one or two fiberglass mini-fifth wheels on the market that fit this weight class. In today's economy, this is the low-cost way to pull something stable and manageable.