The seal on older Class B's

Check for a place that can do a Sealtech test. Essentially this is where a fan is put in, everything else is taped off, soapy water is sprayed on the outside, and it is noted where bubbles form as a potential water ingress point. This will give you definite knowledge, far better than finding out when it is raining.

As for class "B"s, I notice that very few are rendered to scrap due to leaks. I would ascribe part of it because the factory metal roof is a lot more durable than fiberglass or trash-bag thin rubber roofing materials, as well as the build quality tends to be a tad better.