I also disagree. I think you either misunderstood or it was a salesman trying to hoodwink you.
The biggest depreciation hit is when you drive it off the lot. If you buy new and sell every 4-5 years, you're going to get that big hit each time. That is NOT a smart financial decision.
If you buy new, you're better off keeping it as long as possible - the longer you keep it, the more likely what you owe on it will be less than what you can sell it for.
There are three main reasons people trade in an RV after 5 years or less:
1. They bought the wrong RV in the first place (didn't like towing, not enough space for everyone, and so on);
and
2. Their life situation changed and what worked before doesn't work now (got married and had kids, divorced, etc.);
and
3. They are afraid of not having a warranty and having to do maintenance work themselves.