I second that. That's pretty much what I did. I sold my RV and will be using the proceeds to buy my boat. It might take a while to find the right buyer but you will and you might even get more than you expect.
Seriously... think about it... What is the number of people owning a boat that is reasonably fast over 30' LOA... then think, what percentage of those want a 1973 Ducati 750 GT motorcycle, and are willing to trade their boat for it. That number is a lot smaller than the first one. IMHO, anyone looking to do a trade is basically being foolish about it...because you drastically limit your choices and destroy any bargaining power you would have otherwise.