I'm not sure you are going to find any single source to help you. I've never searched for a motorhome, but when looking at other campers, all you can really do is search the internet for models that you are considering and see where the prices fall. NADA prices are generally considered to be on the high side. For whatever specific model I would be interested in, I would check out new model prices and then look at used models listed. I believe most folks expect at least a 20-30% discount off list prices. But with used units, condition of the unit can be a prime reason for paying more or less than average retail. In the end, if you find the unit you want and feel you are getting a fair price...then buy it and don't look back. Here on the forums, you'll always find someone that has gotten it for less...but it's senseless to spend time looking back!
The other factor to consider is the cost of traveling long distances to look at or possibly purchase an RV. And if you are purchasing a motorhome, there is so much more to check out and evaluate.
Good luck and hope you find a gem!
