My problem with NADA is that it is based upon a depreciation schedule and fails to account for other factors such as build quality, desirable features, etc. Therefore it fails to reward things like innovative features, or punish unpopular design choices. For example bunkhouse diesel pushers came on the scene 7 or 8 year ago, and have been very popular, yet the NADA values for these models are the same as their non-bunkhouse brethren.
Also keep in mind that asking price and selling price can be two very different things, even on seemingly good deals there is often room another 20% in negotiation.
Also keep in mind price and age do not have a direct correlation, maintenance, storage conditions, etc. have a lot to do with it. Though while some things age out, batteries, tires and other rubber parts. An 8-10 year old coach that has been well maintained, kept in covered or indoor storage, in a salt free environment may well be in better shape than one that is half the age that has spent most of its life in the sun by the beach.