Damp in a caravan is a "random certainty" - a certainty because it will happen sooner or later - and random because it can occur at any age, even in brand new ones.
A bit like MoT tests on cars which is only valid at the instant it's tested with no future guarantees - a caravan that's tested and shows no damp is fine at that instant but who knows what will show in the future.
Buying a caravan that's already got damp doesn't make sense to me - the full extent may not be visible so any repair might only be partial, in practice - and there's the possibility of a rogue dealer who takes every shortcut, they certainly aren't all like that but some are!