As most have said, there is really no hard and fast rule for depreciation on RV's. There are so many variables that it is difficult to nail down a value. Even on new rigs there's a difference in "discounts" that are offered. It is not unusual for someone to buy a new Entegra and get close to 30-35% off list while getting more than 28% on a Newmar is rare. Add to it swings in fuel prices, availability of campsites and the list goes on.
RVing is a lifestyle, not an investment. If one does not think they can handle the declining value of the rig of their choosing then they should consider other options for their travels. Be careful of the salesperson who says "It's only $X per month." That's true that it's only so much per month but it's that price for what may be the rest of your life.