TPO and EPDM roofs suffer from the same problem... seams that are right on areas that are blasted by water. A fiberglass roof rolled over the sides (as pointed above) won't have water at high speeds (60+mph when on the road) hitting directly on places where it can come into the rig.
With the caulked seams of a TPO or EPDM roof, your motorhome essentially has an expiration date stamped on it due to water getting past caulk and rotting areas like the cabover. The only real defense against certain conversion to scrap by a rubber roof is a spray on elastomer epoxy like rvroof.com or Rhino Lining's, and those dealers are very few and far between.
As for ron.dittmar's suggestions, I would add a 1.5 in there. A one piece aluminum roof that is cupped over the sides by at least 2-3 inches. A friend of mine has a rig that has that, and his trailer has seen virtually every type of weather that can be thrown at it, and no leaks yet.