I'll swim upstream here and say "pin box." The spring friction on the stiff, solid axle suspension on a standard fifth wheel dampens the bounce (which is what shocks do). Shocks will not "soften" the ride at all - that's a spring function. A dampened pin box, however, will make a big difference and make things much gentler on the truck, the trailer and you. I say this as someone who has towed with and without both a dampened pin box and shocks (now have MOR/ryde independent suspension on the trailer).
Rob