It's metal, some of the cut edges, rivets, and fittings not finished and exposed to weather. If they get a combination road or coastline salt, and moisture, they will corrode. There are various protective products that can mitigate it. It's manageable, but takes regular inspections and treatment of affected areas.
All RV's probably leak, and most people don't know it until there's damage. If it leaks it will most probably get to the plywood subfloor (under the vinyl), and left unattended may eventually rot the wood. Again, regular inspections (probing the floor with a moisture detection meter) and prompt repair will mitigate the problem, if there is one.
None of this is difficult, in fact very easy. I inspect ours quarterly and treat for corrosion; have found four very minor leaks (awning lower mount rivets, entrance door loose hinge screw, and improperly snugged rear window opening latches). No corrosion at all after two years, about 14 months travel and living in it.
Watch some new dealer trailers they may have been already exposed to corrosive conditions without maintenance, if they have been on the lot for some time. Then you're already behind the curve.
The Truth is More Important Than the Facts