Not sure what you expected trading in?
While you make think the dealer was "low balling", in reality they must offer less than what they think it may sell for. Otherwise they would go out of business in a hurry.
Yes, the dealer "makes money" on the sale of a new unit to you, but they must in turn make money off the sale of the unit you traded in.
Even a "lightly" or "gently" used RV isn't worth as much as what you paid for it. Used is used, it will never be new again and the price that someone is willing to pay for it isn't going to be close to what you paid for it.
RVs are a lot like boats, insert money and flush..
If you want to get more than what the dealer offered, then try your hand at selling it yourself at the price you value it for and see if you get any takers.