They're offering you a huge trade in price and not marking theirs down. You won't get it both ways. The numbers on your trade-in sound way too good, hence the high price on the new coach.
This isn't hard. Look around at other coaches like yours and see what a REASONABLE trade in value will be, keeping in mind the dealer doesn't pay retail. 30% off the new one is unrealistic. Figure it at 25%. Do the math and see if his final number matches yours. You're going to pay a price for trading in a coach that new....be prepared to eat a little and do the deal if you want the new one.