Climb on top and cover every inch of it on your hands and knees looking for soft spots. Cracking or popping sounds indicate bad wood from water leaks. The number one issue with Rv's is water leaks. Look in every corner, cupboard etc for any indication of water. Look for repaired ceilings or touched up paint. Feel real close around the ac unit , vent, stacks and other areas on the roof that have had the membrane cut during installation. Crawl underneath and look for any impact damage or broken frame welds. Pay attention to soft spots in the floor. It looks nice and I dont think they are way off on the price but it is Craigslist so if it checks out offer them less.