RV Shopping for a beginner - What to look for?

What you see in that picture could just be from accumulated dirt on the roof. Anyone serious about selling would have that cleaned up. I would want to see what condition the roofing material is in, the sealant on the roof seems should have no cracks and the plastic vents on the roof should be flexible and not brittle. Look for ceiling staining, evidence of water in the light fixtures, rust on the upper cabinet metal hardware, peeling or delaminated wall boards.