When researching values of used RVs, especially ones that are only a year or two old, you have to somehow figure out what they actually sell for, not what some dreamer wants to get in their advertisement. Even if you can somehow find a "selling price" you would have to determine if it was artificially inflated to complete a sale. Examples would be a rig worth 50 thousand being "sold" for $70K so the selling dealer can show paperwork giving the buyer $30K (the amount of their payoff) for their trade in worth only $10K. Also, there are "sales" where people with bad credit take over payments on a rig. Often these "sales" are done without the approval of the original lender and 6 months or so down the road the original owner has a huge mess to contend with, but at the time of "sale" they can say they sold it for what they owe on the rig.
Since new vehicles generally qualify for better financing, have an intrinsic value by being new (no one has slept in the bed, pooped in the toilet or farted on the sofa) and can be configured to exactly meet the wants and needs of the buyer, they have much more value than even a one month old used rig. Obviously there is no exact formula to determine depreciation, but an expectation of depreciation exceeding 30% of the new vehicle's actual true purchase price on a one year old rig wouldn't be too far off, and similar to an automobile, three year depreciation in the amount of 50 to 60 percent should be close to accurate. As you get older, condition comes more into play and a well maintained rig has the depreciation curve flattening over the next several years.
Like others have said, you don't buy a RV (or boat, plane, car, truck, motorcycle, bicycle or pair of walking shoes) with the expectation you won't lose big time on the purchase.