You aren't going to get an accurate number down to the penny.
NADA, For sale ads, etc...get you a rough idea but easily can be 20-30% off from actual sales prices.
Problem with doing a trade is you want the final price after they include the trade. It's the oldest trick in the book to offer you a high trade-in value and then they don't give you as big of a discount on the new RV, so they make up the difference.
Push them (even if they offer more than what your research suggests is a good price) and be willing to walk away (without being obnoxious about it). If you do that at 2 or 3 dealers and no one budges...you probably have the best deal that is available.