The stiff overload springs would only make for a harsher ride if they were engaged and there was no load on them and this is never going to be the case. A 4,000 lb. load in the bed along with the load from the trailer tongue is going to need stronger support.
Going to something that shortens the distance that the springs can travel is not going to make the ride smoother. Air bags may provide more shock dampening but then the truck's shocks are not going to work very well and overall handling is going to be compromised.
I don't understand why people expect a heavy duty pickup truck with a maximum payload and its heavy duty tires and heavy duty shocks, should have the ride of a sedan. I have a GM with the independent front suspension which helps but it is still a "heavy duty" pickup truck and it rides like one.
Want a smooth ride then get a 1/2 ton pickup and a popup camper or better yet an SUV and pull a trailer. There are always trade-offs and with any choice you have to take the bad along with the good.