There is no typical discount for used rigs. It's a wide open market, with lots of local variations. Not at all like used cars where values are pretty much the same nationwide. You just make an offer and see where it goes, like the proverbial Middle-Eastern bazaar.
The online NADA RV Guide is estimated depreciation, not based on actual sales data. It's a ball park figure only. You can find recent actual sales prices on the PPL website, but those are consignment sales in the south Texas market, so not necessarily comparable to dealer lot prices or other places.
PPL RVs Sold