Assuming you were bidder 16 I would offer her no more than 2000. In fact your bid of £1850 was the highest if bidder 15 pulled out.
Bidder 15 has not bought the caravan for whatever reason.
As he did not do so his bid of £2500 should be discounted, as you would not have had to bid so high to try to beat him. if he had not left such a high bid in place. If you like the van explain this to the seller and make your offer.
You,ve also got to question some times when a buyer pulls out if it was a shill bid trying to up the price.
Try to get a rough idea of its value if you see the same on here
http://www.autotrader.co.uk/CARAVANS/caravans.jsp
There are some newer same make vans cheaper.