First off ask for any service reports including damp reports the dealer has .Often the damp report may only have 'ticks'which doesn't tell you much. Look at the front windows , particularly behind the curtains, inside the cupboards some caravan wallboards turn a light shade of blue if damp Also look for tiny pinpricks where a damp meter has been used. Around the windows lift the inner lip of the black window seal and look at the plywood colour if it black then its damp and starting to rot. More modern caravans have woodless' walls and roof but the floors may still be plywood , look underneath the front and rear corners , looking for spongy or damp wood( to the touch). Look at the front and rear panels ,where they attach to the sides , looking for hairline cracks.Test all the systems don't take their word and get a warranty off them in writing. If in doubt walk away , its a buyers market at the moment