Al,
Nice unit- we are over the border in northern N.H. We have a 2012 855S
I have noticed some of the same movement- especially on a certain road in the Catskill (NY) mountains. That particular road does it to any truck, SB, LB or box truck.
One thing that I am changing this year is removing the 2 bed mats that the TC sits on. I did that because on NNBS GM trucks the tailgate opening is rounded at the lower corners and I needed the clearance for the bathroom "bump-out". I think that the rubber gives too much and contributes to the movement.
Best of luck with your Lance.
John