20 years is a pretty far reach back to determine a value for a car, nearly impossible for an RV. Be careful using NADA as a standard, better as a rough budget guide. There just isn't much in the way of sales data reported to determine an average with current late models much less something 20 years old.
A bit surprising a dealership even has one that old on the lot. Figured they would have sent it out to an auction yard. Also at that age depreciation has pretty much bottomed out to something less than 1% a year. Might even be increasing a bit at this point.
