Yes, the entire structure constantly flexes and moves, particularly when driving. The static flex is only a fraction of the dynamic flex.
And loads are very much changing from empty/parked to (for example) full black and full gey tank and full fresh water tank, and heavy loads hanging off the rear bumper getting thrown up and down with 2 or more G gorces (120 gal of water, times 3 G (3 times gravity)= you do the math.
BTW: pre cambering does not increase the stiffness (bending resistance) of the beam; it is more a pre bending to end up with a fairly flat beam when some of loads have been added.
