We looked into doing this, and both the dealers in our area offered us an insulting deal. Bottom line is that they will never end up having to give you money. The best you could hope for is an even trade. coolbreeze01 is right. Selling is the best option.
You won't come out very good. The one you get will be priced at retail. The one you trade in will be priced at wholesale. Buy low, sell high. That's the only way they can stay in business. Better off to sell yours outright and then go find the one you want.
2011 Big Country 3250TS...2010 Ram CC Laramie 4wd Cummins
15k Super Glide, Firestone Ride Rite, TrailAir Tri-Glide
Michelin XPS Ribs. Just say no to Chinese tires.
Twin Cities Mn.
Not a direct answer to your question, but we sold two TTs on Portland Craigslist quickly - I see you're in WA, so your Craigslist may be as active as well - that would be my first choice to sell, then go to the dealer with cash in hand. USe the NADA Guides to help you price the rig you own now and be very detailed in your CL ad - that'll keep the phone from ringing with questions, and show people that's you're serious.
It's not always as bleak as other posters have portrayed. Sometimes it's how desperate the dealer is to make a sale.
We bought a new TT in 2007, at an RV show. The MSRP (a meaningless number, I know) was around $22,000; the "show price" was $14,500. Three years later, we traded it in on a smaller unit - the grandkids weren't interested in camping with us old folks any more so we didn't need a bunkhouse model. The 2010 (a Jayco) had an MSRP of around $18,000 and was on sale for $17,000. The dealer gave us $13,000 for our trade.
A week later, our old rig was on the dealer's website priced higher than what we paid for it new.
"The great object is, that every man be armed. Everyone who is able may have a gun." - Patrick Henry