Payload ratings on diesel 3/4 tons are low due to high curb weight and the 10k max class restriction on that series of trucks (for lower registration, avoiding DOT hoops if commercial, etc. - nothing related to capability).
My take on this issue is if buying new and planning on a fifth wheel, buy a one ton. If you already own the 3/4 ton, spend a couple hundred bucks on bags or timbrens and load it up to the equivalent year 1 ton SRW rating for your truck assuming the wheels and tires are also the same.
