Towing capacities aren't realistic, completely unachievable

As to the fifth wheel weights before moving to a dually, it is more about the actual pin wt. Some 16K FWs, depending on how loaded, slide arrangement, axle location, could have less pin wt than a 15K FW. As long as you are within your RAWR/tire/wheel rating, with a SRW, you should be OK..."hook it up and go".

Jerry