I have an older SuperDuty F250 diesel with the Tow Package and the same springs as the F350's. The single rear wheel F350 truck has a larger rear axle tube than my truck and larger bearings. Dual rear wheel F350's have much more gutsy rear axles and huge bearings.
I'm comfortable towing a 12K dry weight fifth wheel with a 2K Pin weight. Any larger fifth wheel, and dual rear wheel 1 ton trucks are warranted.
My 3/4 ton truck is really a bastard truck--without the ability to carry the Pin weights of many very popular fifth wheel trailers with 3K Pin weights and 13K-14K total weights.