I bought a 2001 Newmar Mountain Aire a few years ago and put a fair amount of money into it since then.
I assumed that the diesel engine, drive train, and general coach structure would be good for another 10 years, but there are other things I replaced/upgraded that were getting "long in the tooth" after a decade of use . . .
- replaced both original AC units with new Penguin IIs,
- replaced original inverter with a Magnum,
- replaced the fridge cooling unit,
- installed a new hot water heater (got a bigger one),
- upgraded the water filtration system,
- installed a large solar system,
- lots of maintenance issues: fixed roof, new carpeting, etc, etc.
Much of this was preventative -- not all this stuff broke. I just figured it was time to replace before we hit the road full timing.