You've probably realized this by now, that NADA is not as an 'exact or accurate' method of determining the 'value' of an RV - as say Kelly Blue Book is on automobiles.
IMO, it gives you a baseline to start with, but it is at best + or - 20-25%. Also IMO, the higher up the coach food chain you go, the harder it is to obtain a reasonable NADA price point.
In my research about 6-8 years ago now, looking at higher end coaches it blurs more. And becomes very subjective. Why? Well from my personal observations, quite often the older higher end coaches, have had much more higher end customizations made to them.
What a seller will sell their coach for, and what a buyer is willing to pay for a coach - determines the actual value. (Pretty obvious statement. But NADA is not even close on many of these higher end coaches exchange of ownership prices.)
Also IMO, you have two kinds of owner and coaches. 1) Those that are 'get in and drive' owners, with 'use them as they came out of the factory' coaches. 2) Very involved with maintenance and care owners, with 'I've customized them to how I want the coach to be' owners. Customization? Improved mechanical: Engine upgrades, suspension upgrades, cooling upgrades, expanded battery banks/solar panel, etc. Improved house: Updated electronics via TV's, stereos, Silverleaf, GPS, WiFi/Cellular Amplification, Inverters to PSW, Residential Fridges, carpets, wood flooring, tile flooring, fireplaces, window coverings, cabinet storage modifications, etc.
These costs can be easily from $10 - 20K. And $40, $50, $60K + modifications are also very prevalent.
I also saw two avenues of selling: 1) Ebay/RVT/Craigslist, and 'move them out' consignment locations (PPL for example.). These by owner, or 'move em out' consignment's - are usually at a lower cost sales point. Probably closer to NADA values. 2) Higher end and heavily modified coaches, seem to be also either sold by owner, or higher end Consignment shops. Premier, California Coaches, others, for example.
After I looked for just under a year, and spent about $3K and about 6 days of vacations to go visit and look at more distant coaches that turned out not to be what was advertised. I elected to retain the help of a Professional Salesman. I asked here on IRVE, RV.Net and Escappees for recommendations. And ended up working a gent in my own city of San Diego. Steve Wilson from Holland Motor Home. (No longer with us, as he lost a battle with cancer.) The common thread between the Professional Salesman that were recommended to me, was a true desire to match the needs of a customer with an appropriate coach. More concerned about doing this right, then just making another sell. Why? Well, repeat business and referrals were the major reason why it was more important for them to help a buyer, then just make a sell. This paid off in the long run.
I remember a comment that Steve Wilson made to me on my second sit down with him. 'The good coaches are quite often never advertised. The salesman know of people looking for that certain type of coach, and put the two together.' For example, my coach was coming in on consignment at Holland Motor Homes, Steve called me and told me about it, and when it was due into the facilities. I met it at the gate as it came in. Not cleaned up or prepped for show. I spent two house alone with it, crawling over, under with with a flash light, and thru the full coach. It met what I was looking for, so I took the next steps in due diligence of reviewing all maintenance records (All of which had been done by Holland Motor Homes, so that was easy!), took it to Cummins for dyno tests, fluid analysis, and full inspection of the chassis (Sure, all on my dime.).
Another example, as I was in working on the offer with Steve, he asked if he could take a call. A former customer called, the husband had died, and the wife wanted Steve's help in selling a Country Coach Affinity. When he got of the phone with her, he commented that he knew exactly who to call, as he was helping another buyer looking for such a coach. (Higher up the price point budget wise then we were. He figured this was a $250-275K value, we were shopping in the $150K range.)
A long story here, to show that I do believe that many of the best coaches, never make it to market
OP - Keep doing your research. And sure NADA is a guideline, and your numbers could show trends for specific makes and models in relation to depreciation. Also, if planning to look Prevost. Going the Prevost Owners User Group, and start reading and asking questions. A wealth of knowledge.
Best of luck to you, and all,