There are a lot of variables here, how do you plan to use the coach (vacations, full time, snow birds, etc.), how was it previously used (snow birds can put of a lot wear on the interior of an RV, and few miles on the chassis), how many years of service do you expect to get out of it, ...
With that said, all RV's depreciate, all have an ongoing cost of ownership which includes maintenance, storage, and insurance. The depreciation curve on motorhomes flattens out considerably after they get to be 10-12 years old, older than that and the price becomes much more about condition. Under the umbrella of condition, I am including not only the physical shape it is in, but also mechanical features like which engine and transmission it has, as well as styling, purple and teal accents just scream 1990's.
As to mileage, it is very rare that one will find a used motorhome that has so many miles on it that mileage itself will be an issue, unless perhaps you plan to go full time and travel back and forth coast to coast repeatedly. Generally modern (anything built in the last 15 or so years) motorhomes will be equipped with engines that have design lives of 200,0000 (gas) to 400,000 (lower end diesels) miles or more. What is important is what maintenance has been done, have those components that age out (anything made from rubber) been replaced for tires that means they should be less than 7 years old, belts and hoses about 10, suspension and steering rubber bushings, should be inspected and changed as needed likely after 10-12 years.
For me personally knowing I would likely be taking one big trip per year, and the rest would be long weekends here and there, I opted to buy something in the 12-15 year old range when I bought my current coach last year. I paid $20,000 for the coach, have put another $6,000 into it in maintenance and upgrades (mostly upgrades), and feel based upon current market prices for the same model that I have, but 5 years older, that in 5 years I could sell it and only take a $3,500 - $5,000 hit. If I would have bought the same basic coach, but 5 years newer which would have been a 9 year old coach (of course with a few updates, 6 speed vs 4 speed transmission, etc.) I would have paid around around $60,000, and in 5 years it would likely be worth $25,000 or so. Which is a loss of $35,000 in value over 5 years.
What would that $35,000 have bought me, a little better performance, though likely less than 1 mpg better fuel economy, a more contemporary color scheme on the exterior paint, a flat screen TV straight from the factory vs my retrofitted TV with cut down wood cabinet., oh and a curved glass shower door instead of my neo-angle glass shower door and maybe a little more contemporary faucets and bathroom fixtures as the polished brass look on my bathroom fixtures is a bit dated.
8.1 Vortec V8 engine, both have it
DSI electric water heater, check both have it
Sharp Convection Mircrowave, check both have it
......
LED lighting, check both have it, mine has been updated
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