You used to hear 30% and 35% bandied around frequently. One thing you can also do is to use www.RVtrader.com or www.rvt.com
Use their advanced search and feed the information for exactly what you are looking at. It will then show you ads for identical models and the prices advertised by dealers. You will find a wide variance in those prices, and will also find many variables in how the dealers transact their business. Some will low ball a price, and then add thousands in fees and charges. Some will price their units straight out the door. You can go tho the individual dealers websites from there and see their methods, no fee straight up dealers tend to promote heavily on their business models, dealers with a lot of add ons tend to not show much if any information on their sites.
CW uses every ADD on or fee they can dream up.
Also remember, these units have wheels, you can buy one from a hundred or more miles away.