While some people and their habits (smoking, pets) can be downright stinky, the odor you are referring to is caused by water intrusion pure and simple. We have become conditioned to think of water intrusion as a leak (especially roof) but it can be drips under a sink or dampness in a mattress that is never aired out or flipped. I once spent the night in an Air Force barracks in the Azores where the mattress was soaking wet from humidity.
When looking at a used camper, that musty smell is a hint to check for leaks, feel the carpet and check for soft spots. If you don't find any, then a simple airing should be sufficient to clear any mustiness out.
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