Key to all this is whether you owe more than the value of your trailer. If you owe more than the dealer will give you then he will take that unpaid balance and deduct it from the MSRP. That's how you get your unit paid off. The dealers using his profit to pay off your unit. The key is how much if any is owed above the value of the trade in. If the trade in is paid down to the trade in value then you can work with the 20-30% profit margin of the MSRP.
Or you can pay down your unit on your own and deal with the inflated MSRP and try to get it 20-30% lower.
JMO but it's hard to get 20-30% off MSRP if the value of the trade in is lower than the trade in allowance. The difference has to be made up somewhere.