Respectfully, Wayne, yes they do bounce up and down. They may not "flop around" but they do bounce up and down every time the truck bounces up and down. It may not be visible to the naked eye but there is extra stress being put on the mounts with each and every hoop-de-doo in the road, each and every pothole. It's all about inertia.
If the mounting is not adequate, such as if the generator were strapped directly to the ladder, imperceptible motion would slowly become perceptible motion, then "flopping around" and finally the ladder with generator laying in the middle of the Interstate somewhere.
