A search of the Forum may turn up old discussions of this.
Briefly, when new, mine cracked slowly after being winterized by the house for several months, and I ultimately found it was from a twist in the coach, amplified by a (common) factory mis-mounted windshield. Our gravel streetside rear pad area was 2" low, but aggravated by a leaking air bag fixture, the coach over time tilted to the rear left corner.
Releveling the pad side-to-side and dumping all the air before storage did the trick. The pad could be off front to back, given the front tires are on fairly level concrete; but it couldn't be off side-to-side much, and I hadn't been diligent there. Dumping all the air mitigated any air leak effect.