They don't have to have a leak in order to grow black mold. Moisture from cooking and showers is enough to start the process. Molded fiberglass trailers can still leak through any opening if maintenance is not done, the windows, the ceiling vents, doors, air conditioning, leaks from toilet, sink, shower, etc. Carpeting can easily develop black mold from moisture in the air. Casitas do get black mold, check related forums.
One does best in a dry climate. In a humid climate, there is a constant battle of checking and treating the mold. I don't remember ever seeing black mold in the olden days. I suspect the new materials feed the black mold issue. Black mold will even grow on dust, so dust needs to be cleaned frequently, not my favorite thing.
There are ways to deal with the moisture for better results. There are some different ways to do this.
If you purchase a used unit, check carefully for leaks. I would have an RV shop/technician check as they know where to look even if the previous owner has attempted to disguise the problem.