The depreciation is worst the first year of ownership, a little less the second year, less than that the third year, and so on. The longer you keep it, the less depreciation per year. You have to balance the depreciation cost with condition and reliability, but buying new every three or four years is going to cost more money than keeping it longer. Plus in some states such as CA that triggers a whole new round of sales tax and higher registration costs.
My thought is if you want new, for maximum value buy something well built and hang on to it for ten+ years or till you're tired of RVing. If you maintain it halfway decently it should last. They're not getting the mileage accumulation of a typical car. Additionally after the 10+ years you may be getting a little tired of RVing, but still enjoy local trips. An older rig is perfect for local trips.