Frame Stiffing to reduce Porpoising?

I get concerned about our camper porpoising sometimes as well. It looks like the cabover is moving a ton when I look up and out the windshield. But in reality, it can't be moving too much or it would be hitting the sharkfin antenna. And, since ours is sitting on a rubber mat in the bed of the truck, and the center of gravity is right on the rear axle, the flex in the rubber mat probably accounts for a lot of the movement. As mbloof said, 2" of movement at the front of the cabover isn't very much movement at the front and back of the truck bed.