They will take boats in trade but they will only give you wholesale price for them. They will typically call around and see if they can find a boat dealer to buy it from them before they commit to a trade. If you know that going into the deal and you're o.k. with the price they offer then go for it. The problem with this time of year is that most dealerships won't want the boat for the same reason you don't, because it will sit all winter without selling. Just keep in mind there is a "winter" selling price and then there is a "summer" selling price and the two may be very different unfortunately!
Good luck and I hope you enjoy your new RV!