Most will recommend at LEAST a "3/4 ton" (ie F250, Chevy 2500 or the equiv) to pull a fifth wheel. It has more to do with frame rigidity and truck weight than towing capacity.
You'll see many threads on this, but as a card carrying member of the newly appointed "Tow Police", I feel compelled to pass this on. Most half tonners aren't up to the task SAFELY. Can they pull it down the highway? Of course. Can they control the trailer in a strong crosswind, a sway condition, or down a steep grade if the trailer brakes fail? Maybe....... Remember...this is the safety of your family we're talking about here. Serious stuff. And few things are scarier than having the tail wag the dog when a semi passes you or watching the ditch get closer when a strong gust of wind hits and knowing there's nothing you can do about it.
Find out your trucks GROSS COMBINED WEIGHT RATING. Should be in owners manual for your cab, engine, transmission, and rear end combo. Take off 10% immediately if you tow out here in the steep stuff....big mountains, etc. Now take that number and subtract out the weight of your truck, passengers, fuel, luggage, camping stuff, and anything else you lug around. The balance is the upper limit of what you can safely tow. That doesn't mean you can do it comfortably, just safely.
Remember, the closer you get to your trucks limit, the less fun towing is. As you approach the limit, towing in the hills can turn into a white knuckle freak show quickly. In most practical terms, there is no such thing as "too much truck". An F350 Powerstroke dually will gladly pull along a 3000 lb 18' trailer.
I too see a lot of half tonners pulling decent sized fivers. Many of them, IMHO, are one piece of bad luck away from disaster. Hope this helps.