Go to the dealer without your trade and do not mention that you have a trade in.
Negotiate what you consider a good price for the trailer you want to buy. DO NOT MENTION THAT YOU HAVE A TRADE IN! After arriving at what you consider a fair price for the new trailer, then and only then bring your trade in into the equation. Ask the dealer or go to a library and get the wholesale value of your trade in from the NADA RV pricing guide. This guide is not the NADA online guide.
Subtract the wholesale value, from the NADA guide, from the "no trade" price you arrived at with the dealer. The difference will be you price w/trade.
The paperwork will show the full MSRP price of your new trailer minus a greatly inflated value for you trade in. But, the difference will be the same as the procedure I outlined above.
If you want more than wholesale for your trailer sell it yourself, but remember you'll pay full sales tax when you purchase from a dealer without a trade in.
This procedure is also the same when purchasing an automobile. This is Consumer Reports method of purchasing vehicles WITH A TRADE IN.
-Tom