As was mentioned, you can turn tight enough, (usually backing up), to hit the trailer with the truck bed. You might be O.K. going down the road but you want to check when backing into a site. A rack would be a solution. I have a cap on the bed with racks and I put another bar on the cab. I can carry two canoes with this arrangement because I can move them far enough forward, over the cab so they don't extend out very far in the back. When I carry one canoe, it's centered so it's position doesn't change relative to the trailer when turning.
If you don't have a cap on the bed, you can get racks that mount on the bed rails.
