GM pickups have much less flex than a Ford, you can search frame flex videos and see the drastic difference. A standard rubber bed mat will also provide cushioning while helping hold the camper firm, the problem lies in many go cheap and heavy by using horse stall mats that are three times thicker and offer plenty of rocking potential. My setup is a standard rubber bed mat and an aluminum superstructure camper with minimal movement up top, but it still does move some.
Fords were flexy until the 2017 Super Duty chassis. Before that they still had a c channel frame where Dodge and GM went fully boxed in the early 2000s new models.
