Notice in the photos that the roof is a flat one .... that is not the best. You want a roof that is crowned (higher in it's center and lower along it's side edges) and made from a single piece of fiberglass.
Even if the roof is crowned and made from a single piece of fiberglass, it should be rolled over all along it's sides so that the motorhome's sidewall pieces of fiberglass fit up underneath these rolled over roof edges for the ultimate seal along the sides of the roof.
From the Craigslist photos, you can't tell if the roof is fiberglass or some other material - but it is obvious that it's not crowned and does not have rolled over edges.
For example, study the front and sides of the single piece fiberglass roof on the first page of this brochure ... you can also see the crowning and rolled over edges by zooming in on Page 9 of the brochure:
http://www.winnebagoind.com/resources/brochure/2005/05-Spirit-bro.pdf