Around £1,000 depending on the size and make of the caravan which you failed to mention. At that price if old c/van look at buying another take damp meter with you to test the inside walls for damp especially near windows. Also ensure you check the plate on the side of the c/van either by the door or on the tow hitch about 3 x 2in give year of c/van look up on internet 'how to check the id plate of caravan'
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix caravans/motorhomes.