I don't think anybody has been making them long enough for us to find out how long they last. I've seen one of the earliest Newell coaches, more than 40 years old, still in use for full-time RVing, with the original owner, who started very young. There are Flexible and GMC coach conversions still in use after 50-60 years. I regularly seen early Winnebago motorhomes 40-45 years old still in use.
If you are starting at normal retirement age, almost any motorhome, C, A gasser, DP or motor coach conversion, can last longer than you will. It is strictly a matter of care and maintenance, how much money you will continue to put into it, as opposed to spending the money on a new one because you want a new one.
On the other hand, a careless owner can turn a $2,000,000 motor coach into s**** in less tha five years, not including collisions or plunges from a mountain road, which can s**** it even more quickly.
Really not much different from cars. I've worked with guys who kept the car they bought when they finished high school or college through their whole working life, then kept it as a classic after retirement: fifty years or more. I couldn't do this, grew up in Michigan, 1950s cars were rusted hulks after 3-5 winters, 1970s cars lasted no more than 10 years. So out of habit, I tended to keep cars in service 8-12 years, even after moving out of the rust belt. My motorhome is now 12 years old, decals fading, plastics yellowing, but a few hundred dollars a year taking care of things as needed, keeps it as functional as when it was new. Keeping it pretty? That would be a lot more money.
Tom Test
Itasca Spirit 29B