Our 95 developed a couple of small cracks on the lower edge in the middle of the roof when it was 15 years old or so. We decided to replace the top. The new top lasted a year before the first crack zipped right across in the middle. I had it and the ensuing cracks welded up until it started cracking alongside the welds and it developed a crack across the underside. It was at this point I replaced it with a fiberglass top made for me by Colorado Camper Van.
Causes of cracking. The cracks in the original top were most likely a combination of age and winter snow load. The second top is more problematic. After installation, I put GoWesty-bought Yakima roof racks on. This entailed drilling holes into the roof for the rail anchor bolts. Some of the cracks that developed later (but not all) ran through these holes. That still doesn't explain the crack across the inner shell, though.
Lessons learned. All these cracks were expansive in nature -- that is, there was empty space between the crack edges. So there may have been stress applied to the outer shell by the metal frame inside the top. Perhaps one should avoid drilling holes in the top and garage your bus or live somewhere that doesn't snow a lot.
Our new top is so superior in the ease of use, the extra space, and convenience during weird weather that it would take a few more paragraphs to explain it.