I admire your honesty and integrity, too often I have to bite my tongue when reading on this and other forums of people who have problems, damp and otherwise, and just trade the caravan in against a new one. The general attitude seems to be "as long as it's not my problem anymore". If the supplying dealer hasn't been able to resolve the problems in their term of ownership then I doubt if they'll do any more before selling it on to the next unsuspecting soul.
As said above, if you advertise it as honestly as possible then at least people can make their own mind up whether or not it's an acceptable gamble at the price agreed.