My Dad was in the plumbing business many years. I worked for him at one time and the tub manufacturers would not honor the warranty of the tub if any addition blocking was added under the tub. The tub is supposed to flex.But there are some materials that should not be used in RV's because of the flexing that occurs as the trailer moves down the road.For instance, trailer manufacturers use ABS drain pipe instead of PVC because ABS is more flexible than PVC. So not sure what materials you used or how they were installed but perhaps that could be the problem.