Many respondents to this post are (apparently) basing the load capacities of the various sizes of pickups on NEWER models, not what the OP may purchase, as he wants a used pickup. My (2000) F250 PSD has a GVWR of 8800 pounds, and I'm right at its capacity with my light weight 26-foot 5th wheel. Some of the new half-tons can probably carry as much weight as mine, if you go by the numbers. So if the OP is buying a 10-year-old pickup, he'd be best to disregard any thoughts of buying a 3/4-ton truck.
OP, even if you buy new, go for at least a one-ton, and if you can manage a dually (garage space, etc.), that'll likely be your best vehicle for towing a 5th wheel, plus you'll be able to purchase almost any 5er you might like with a dually and not be limited to the lighter weight ones. I bought mine in 2000 thinking it could tow any camper I might want. We'd have been in a bigger camper years ago if I'd have initially purchased a dually -- or even a 1-ton srw.
One other thing, OP. I noticed you were asking about "towing capacity". In case you haven't caught on by now, it's "hauling capacity" that you need to be more concerned about when towing a 5th wheel. Most 3/4-ton and 1-ton diesels will "tow" all you'll likely want, but much of that weight must go onto the truck itself. That's where 3/4-ton pickups often fall short.