Whitsun is the best selling time for both private sales and dealers.
At this time of the year Dealers don't really want to take expensive caravans in which they are unlikely to sell until Spring and that might deflate what they'll offer you unless of course they're keen to sell you a much more expensive van.
It's a good time to buy a new caravan though with delivery around March/April. The dealer has a sale, doesn't hold your caravan in stock over winter and can more easily and quickly get it off his books in the Spring. So he can afford to be a bit more generous.
I did my last deal in October for a new caravan with delivery in March. We agreed a trade in price for the old caravan and a cash price for the new caravan if I sold privately. I managed to sell it privately within a couple of months.