Hi,
In the twenty plus years I have been in the industry I have seen many newly purchased second hand caravans and motor homes that become money pits and worst of all are beyond economical repair.
I cannot stress enough the importance of a service history for a caravan and a service history that includes a habitation check for a motor home, if the vendor cannot provide service history/habitation checks then how can he know the condition of the vehicle he is selling? No history equals walk away in my book, you could regret the purchase in the future.
The history must include the all important damp test, please examine this, take your time to digest the diagram, it can be confusing when first seeing one, take notice of any readings above 15%, if there are any elevated readings ask the vendor if you can inspect closer, push gently with a figure the wall, ceiling or floor if it is soft compared with areas where the readings are 'normal' this confirms the water ingress and could indicate that the leak has damaged the ply and timber, I have seen reading of 80% plus that have not gone soft, these still need to be addressed by sealing the offending seal and drying out or replacing the board and timber.
The service check list should also be available to confirm any other possible faults.
I hope this helps.