I think a lot of people will say that you should aim for around 20-25% as a discount off the MSRP. If you are trading and the dealer is giving you full retail price for your trade then all they are doing is moving money from the profit on the new trailer to offset the "over allowance" on your trade.
What you want to ask the dealer is: "WHAT IS THE ACV" of my trade? ACV stands for actual cash value. Tell them you don't want to dick around with fake numbers on selling price and trade. I would ask them to sell you the new RV for 20-25% minimum below the MSRP and then work the trade value higher.