I don't think they're making coaches now that will last 50 years. I don't think the quality in the production motor homes is good enough and newer, lighter materials will probably not survive the neglect that aluminum and stainless steel did.
Also, it's hard to imagine that the DEF emission engines will last longer than the old coal burners, mostly due to the higher combustion temps involved. And those that are jumping at the keyboard to remind me that truck engines are better now, let me remind you that truck engines don't sit idle for days or weeks at a time.
As to longevity, the large majority of general aviation airplanes are 40 years old and more, The 2 I fly were manufactured in the sixties. The reason (other than new planes are to expensive) is that planes are required to go through an annual inspection where the plane is basically disassembled and put back together with new parts for things that are out of spec. The typical cost of this inspection is $1000-$3000, and is inflated because of the paperwork required by the FAA for all parts. I'll bet most motor home owner spend more than that a year. Of course, planes don't have fridges and water heaters.
Proper maintenance will only get you so far. The initial quality of the coach will be the deciding factor. As with many things, you get what you pay for. Times were people saved up for years and bought a good quality coach. You insisted on quality and you took care of your purchase because you busted your hump to earn/save the money to buy it. It had some value in your life.
Now, with 5% down you can go out an buy some plastic box with a motor - no pain, no real value. You keep it long enough so you're not upside down, put it in craigs list or consignment and buy another.