So appareantly there are too many used 5th wheels

I'm surprised they didn't just make you a really low offer for trade-in. No matter where you try to sell it, be prepared that your's has really depreciated. You will always get really beat up buying new and only keeping it a year or 2. I would hang on to it for several more years and at least spread out the depreciation. Or Sell it on your own but you'll have to price it lower than you want to.