I totally agree with brirene....
Be aware that dealers have their own NADA guides and they are NOT the same as those you have access to here on the internet and their guide will always have lower values for trade in.
Your best bet, of course, is to sell it yourself as you will be able to get more for it than you would at a dealership. However, you've already stated you want to trade in....
i would not take the trailer with you until final negotiations. They will ask you up front if you want to trade in your trailer.....at this time just tell them you have not made up your mind and that you already have someone who is interested but have not committed themselves. Once you settle on a price for the new rig, then tell them the deal fell through and you've decided to trade in. The downside to this, is that you will probably get less for your trade in, but should be off set by the lower price for the new one that you have already negotiated.
It's really a toss up. All depends upon how YOU like to negotiate and dicker. The dealer will probably come out of the deal with the same profit either way.
Good luck
Ron