Motorhomes DO depreciate, largely because they do deteriorate with time. The finish on fiberglass coaches that do not have full body paint, and rubber roofs, deteriorate particularly quickly.
One way to at least reduce the depreciation costs is to buy a good used BUS instead, that has been converted PROFESSIONALLY (not by a well-intentioned but unskilled and budget limited amateur!). Buses tend not to deterioate very much with time, as they were constructed by their manufacturers for a standard life of 30 years or 3 million miles (100,000 miles per year is typical for a commercial bus). The diesel engines in these buses tend to be Detroit Diesel engines versus Cummins or Caterpillar, and they are very robust and LESS costly to work on than Cats or Cummins, as the installed base of Detroit Diesel engines over the past 80 years or so is huge. Parts are cheap because of the volume as well.
I am trying this approach myself right now. My wife and I are moving out of our house and into fulltime bus living in a 1979 Eagle bus that was professionally converted in 1996 and used by the same couple ever since then. When such professional conversions are done, they are almost always reskinned, as was ours, and with aluminum and steel, not fiberglass, and so no one thinks it is anywhere near 30 years old!
In this depressed market, a really nice conversion bus can be bought for $50k to $100k. Ours cost us much less, as our budget is very limited. You can call me if you like at 612-270-7645 if you are interested in this idea at all and want more information.
Jim G