So you started with E rated tires and went up to F and now G's. Before you run those at max pressure look on the inside of one of the the rims for a max inflation pressure.
That would not be your answer for your previous blowouts.
Back to your first set of blowouts. Old tires that sat in one spot for years. Probably should have been replaced before the tow home.
Second set, Marathon tires. I just replaced 4 of them myself that were 4.5 years old. One blew out and two others developed bubbles in the tread.
Third set, off brand tires. Could be the same tread separation issues the Marathons have.
Valve stem, they should be new every time you replace a tire.
My suggestion would be to have some one knowledgeable look at the trailer suspension and alignment.
2021 Chevy Silverado LTZ 3500 Diesel
2022 Montana Legacy 3931FB
Pull-Rite Super Glide 4500