So what do you use in place of those 14"&15" ST tires? And I'm not talking about back in 1991. I mean now, today, pertinent to the original question?
Love the ridiculousness and the "uphill both ways to school, barefoot, in a snow storm" diatribes.
In 1991 as today I upgraded 14" wheels on my job site tool trailers or any type trailer to 15" even today if 14" P class tires with enough capacity aren't available .
In 15" sizes, in 1991 as today, we had Goodyears Wrangle HT LT235/75-15 C or LT215/85-15 D load ranges for my tool trailers with 3500 lb axles.
Also we use 15" P class tires on smaller trailers with 3500 axles and smaller.
Maxxis LT U-168 are a commercial grade tire with several 15" sizes and have became popular with all types of trailer owners....not just rv trailers.
Vanco 2 from Continental another popular tire for trailer users has several 15" sizes and load capacities.
I see the Yokohama RY215 in a 700R 15 D at 2040 lbs capacity is popular on some of the other RV websites.
Goodyear Cargo G26 2249 lb capacity in a 225/70R 15 C 65 psi load range D for trucks/vans and trailers.
On my rv and non rv trailers with 5200 lb axles I upgrade to 16" wheels and LT E tires.
When choosing a better tire in a different size you will have to do your home work on what fits in the wheel well or how to make room for a different size or diameter.
If you have trouble with a fitment I and others on this forum can help you make a wise choice.