Who cares.
A dealership is a business and a business wants to make money. If it can use the NADA to validate their pricing on a trade-in and make money in the process, then that's what they'll do. If you ban the NADA, then they'll just use some other source. Then what, ban that alternate source as well?
Don't get upset because someone thinks your poorly constructed RV that's been rolling down the road for who knows long isn't worth exactly what you "feel" its worth. And Airstreams aren't "over-priced". They are expensive, yes, but the term "over-priced" suggests something far different.