You'll probably discover that finding a Class C with some kind of a one piece fiberglass roof is easier than finding one that also has it's edges rolled downward all along each side.
FWIW, our 2005 Itasca has the rolled edges - 3 or 4 inches of roll downwards over the side walls. We knew nothing about RV roofs when we bought, so I guess that we lucked out. It's not only that a roof built like that will remain more leakproof, period. It also means that you don't have to do much to keep it that way. No leaks with proper maintenance is one thing - but no leaks with hardly any maintenance is a whole lot better. Thanks to forums like this, you don't need to rely on luck anymore to find out about that kind of quality construction.