Do things separately. Do not let the salesman combine everything. Haggle against the list price and get a selling price you are comfortable with. Then do the same with your trade.
Doing it this way will be much less confusing. You will be able to tell if you got good pricing on the camper and you will know if you got a good trade-in price for your old rig.
This is the only way to know if it's smarter to sell your trade outright. And remember that even if you sell your trade for a little less then the REAL trade-in value, you will probably come out ahead because you can go back and renegotiate the deal as a 'cash-only' deal.
Remember - You want clarity, not confusion.
This process is how I always buy my vehicles. The salesman and I decide on the selling price of the new vehicle first, then we decide on a trade-in price for the old car/truck.
Tim