As I noted in my earlier post in this thread, the load rating for my A-frames Chinese tires is REDUCED when used with dual axles. This tells me what I had already had guessed - a dual axle installation is much harder on tires than a single axle.
My guess as to other reasons A-frames and PUPs don't seem to have the tire problems the big boys do is
1) more difficult to overload a PUP or A-frame because the fold-down requirement limits internal storage space.
2) the fold-down nature reduces cross-wind loads on the tires - there's not as much side wall to catch the wind and cause the tire to squirm under the side loads.
3) I think it's easier for tire manufacturers to make a smaller tire without defects. The smaller size Chinese tires can meet specs without as many rejects. Again, conjecture on my part.
4) There seems to be more headroom on the tire loading on the smaller campers. My A122 has a GVWR of 3,068 lbs. The OEM Westlake 185R13 LR D tires have a load rating of 3,420 lbs. That's over a 10% cushion if I'm at max weight.
Since I normally trailer at about 2,800 lbs, I have a real cushion of close to 900 lbs (26%) on the tires when the tongue weight is subtracted. Even a marginal tire should be able to operate quite successfully for many miles at only 75% load, especially if kept properly or fully inflated (I use the full 65 PSI).
I expect the OEM Westlakes to age out (I think I'm going to use 7 years since my A-frame is stored inside, no UV degradation except when towing/camping) before they wear out or fall apart. Typical year would be 2,000 to 4,000 miles. I'm on my 2nd year with 3,500 total miles to date. Time will tell whether I'm right or not.
2014 Rockwood A122
2008 Hyundai Entourage (minivan) with Equalizer 600/6000
camping Colorado and adjacent states one weekend at a time