I towed my 4,500 pound boat behind my 11' camper with a 48" Torklift extension. It wasn't any worse than without the boat. The truck and TC don't really ride that great but the truck is loaded 1,700 pounds over it's GVWR so what do you expect.
I sold the boat and am looking to upgrade to a 23' or 24' that would weigh about what yours does. I will need to go to a WDH since the extension is only rated for 6,000 pounds without one. While I don't look forward to the problems this will cause with the surge brakes I think it will improve the ride.