I agree. Very few 1/2 ton trucks have enough cargo capacity to tow a 5th wheel because they put 20% of their weight directly over the truck axle. Trailers typically will place 13% of their weight on the hitch so you don't need as much cargo capacity.
That capacity is whatever your door sticker says minus all passenger and cargo weight in the truck including the hitch and any weight bars.
Usually you run out of cargo capacity long before gross weight rating or max tow rating. Depending on your truck you might be over with either of those but I would think 6000# dry weight should be doable for most half ton trucks. But not a 5er unless really small.