Guess I’ve been lucky, towed around 40,000 miles I figure in the last two years and no problems. Sounds like you are doing everything right. Only thing I can think of is to get the rig weighed and make sure it’s not overloaded. But more likely a bad batch of tires I would think.
If you’re looking for replacements the brand Maxxis have a good reputation.
I’m using TowMaxx, some folks report problems, I’ve had none. I wore out my first set of load “D” and went to load “E”s that have an extra 2 plys and can hold max 80 lbs air. It did seem to improve the ride a bit and they don’t bulge as much.