I don't have a motor home, but as with used cars, you don't know the history of a used vehicle. All my life I purchased only used vehicles, until we bought our first pop-up camper and realized the value of buying new, with warranties and the known history of the vehicle / RV from the very beginning.
I started purchasing new vehicles and found I really, really, really liked NOT having to deal with repair bills due to hidden problems that unearthed after a short while. Yea, there is a depreciation, but the feel and experience of brand new is marvelous.
If I were to get a motor home (of any class), I'd go for new. Depreciation yes, but at least the first 2-5 years there's no repair bills to deal with. The cost of unforseen repair bills on a used unit could end up costing more than depreciation over the same amount of time of ownership. New for me! As all my trailers were, and now all my vehicles... including my Riding Mowers.