I would concur with others that your best financial deal will likely be to sell the current rig outright. Depending on where you are registering you'll lose the tax advantage but you may still be better off overall. You might see what the dealers for your next rig will do, but chances are you will bleed $$$ on a trade.
What is the issue with the current rig? We looked at one before buying our Tiffin. The issues we had were we didn't want a dishwasher or a residential fridge and we thought the bunks were a bit short. The overall layout wasn't that much different from the Tiffin, just enough.
Good luck!