RVs will sell for what people are willing to pay for them. Unlike a car, no one looks at buying an RV as a necessity. So they aren't going to shrug and say "well I got to have it, so I'll have to pay the going rate".
For me, if the concern about depreciation is that big of a concern, then you shouldn't buy.
Better is to weight the cost against the joy of using it. If it costs you $50k and you use it 2 weeks per year is it worth it? How many years would you need to own and use it to make that $50k worth it?
One year of ownership - you could have saved a lot of money on that 2 week trip and headache of having to sell it afterwards.
Ten years of ownership - that's much more realistic.