I'll have to second camp-n-family's response. Tow rating is a helpful (though not 100% reliable) figure for towing a trailer, but not a 5th wheel, because of where the weight sits. Pin weight of the 5er sits ON the bed of the truck, directly over the rear axle, while hitch weight of a "bumper-pull" trailer sits on the hitch itself, which is (or certainly should be) attached to the truck's frame, and that weight is mitigated by the use of a weight-distributing hitch, so the entire truck frame carries it, instead of just one spot on the truck.
At least that's my understanding! :) Pin weights are virtually always much higher than hitch weights, too.