A great question and some great responses.
From my view there are a couple more issues that effect the depreciation, the first is the original selling price. Many of the new prices are way out to lunch. I looked at a couple units recently and one was listed for $168,000, I pulled some ads from a couple years ago and that same unit was $80,000 so in two years it doubled in price but the chassis that it was on only went up by $5000. Used units were being listed for $120 to $140 and they were NOT selling because people were realizing that the prices were insane. In actual fact that new $168,000 unit was worth closed to $80,000 or less. When I talked to a dealer they told me that they are selling for market value but almost 100% of the inflated units were being financed so really the buyer was falling for the "Only $250 a month" sales pitch and not looking at the true selling price.
Second is the financing. There is NO reason that an RV should be financed for 25 or 30 years with zero or next to zero down but many dealers are playing with the terms in such a way that they are getting 30 years and in some cases more. One local dealer even runs ads "Are your payments on your RV too high? If so come in and we will adjust them"! The way they are adjusting is they are adding big time years on the loans to save the borrower $50 or $100 a month but in the end they are paying tens of thousands more in interest and are facing huge buyouts at the end. Then when the borrower wants to sell they find they owe more than what it is worth. Saw a 5 year old unit that was for sale for close to new price, I knew the seller and he said "I cannot afford to sell for what it is worth".
If you are buying buy what you can afford, sure you may get an older unit or one with less features but if you pay cash or the majority cash then you will not get stuck owing so much if you want or need to sell.