Never trade in an RV. Unlike the auto industry there is no reliable "book" price. For new or used. The only way to know what you are really paying for an RV is to pay cash. And the only way to know what you are recieving for your old RV is to sell it for cash.
I will not knock the RV dealers much for this, as they deal with the same uncertainties in a trade in situation as well. If they misjudge what they can sell for your trade, then they stand to lose money on the deal. Obviously if that happens too often, they can't stay in buisiness.