I never gave depreciation much thought when purchasing my used Cambria. I purchased from a private seller in Mn. and I live in Ca. Exactly what I wanted and his out of pocket upgrades sweetened the deal.
I don't trade off my vehicles very often so depreciation wasn't a consideration nor did I look at my rig as an investment. I give all my vehicles top of the line maintenance and when the time comes I get what it's value is. Even considering the fuel mileage. At 8.5MPG after 30K miles it's still better than any hotel i ever stayed in. I know when the linens were changed, the food is good and it's just plain fun being in it on trips.
I would purchase used. Many of the warranty bugs have been repaired, the value is there because of the depreciation hit the original owners took.
I would not purchase a warranty. A Ford chassis and the V-10 will run 300K miles easy with proper maintenance. Save the money and use it on a reputable mobile RV mechanic. I am a mechanic by trade and use mobil mechanics on some of my projects .
Hope this helps.