You didn't go back far enough to find the mythical "good old days".
Back when I grew in the 50's and 60's a new car came with a 12,000 mile/12 month warranty. Tires were about the same and you considered yourself lucky if you got 12,000 miles out of them. If you ran across a vehicle that had turned 100,000 miles, you were amazed and you did not want it because it was obviously worn out.
Now, a vehicle with 100,000 miles is not that big of a deal and you can buy tires that will age out before they wear out.
Motor homes of the 60's fell in the same category as the automobiles. In other words you were constantly having to perform maintenance to keep them on the road and the mileage was as bad as it is today. They may have used slightly better materials, but the workmanship wasn't any better. The chassis were marginal at best.
Now here we are today with all vehicles. We have come to expect our RV's to drive as well as our automobiles and to last as long as them, with no input from us. In other words, just hop in and go until the "check engine" light comes on.
What needs to be understood is that most RV's are being driven very close to the maximum weight allowable on the chassis, suspension, and tires. This means that all mechanical systems are being used to their maximum design loads. Most mechanical equipment can work at close to their design limits but they must be meticulously maintained to do it for any length of time.
The house portion is another matter. Most materials used to construct an RV are being asked to operate in earthquake conditions every time the vehicle is on the road and still expected to operate for years. The facts are that most of the items in your RV do perform as designed. Naturally some things are going to fail and will need attention.
Because of today's regulations, all of our vehicles are loaded with computers. I have 30+ separate computers in my hemi automobile and would guess that a lot of RV's have a similar number to control the engine, transmission, emissions, etc. Most electronics are fragile and we all know about computer glitches.
Personally, considering the complexities of today's vehicles, I think the RV's are doing pretty good as far as reliability, as long as we don't forget to do the necessary preventive maintenance. I tried take care of my RV's like my airplanes, meaning most things were expected to reliably operate until the next ANNUAL inspection. With the RV, I would tend to replace worn items before they broke and left me stranded. I wasn't always successful, but my on the road issues were always minor and correctable with a minimum of effort and expense.
I'll still take today's RV's and automobiles over any of the vehicles I owned in the past.