I think that the 2500 should settle about 3" without the overload springs. Just a guess though. The springs settle a certain amount depending on it's weight per inch of travel. You certainly do not want springs that do not deflect. The engineering problem is they want them to deflect a short amount, then hold strong.
However I don't ever recommend a 2500 series with a fifth wheel. If yours is smaller and light, it should be fine. Expecially with the lighter gas engine, it will lower your curb weight and increase your Gross Cargo Rating.
Fred.