I have Firestone Transforce HT tires on my truck from the factory.
Just turned over 40,000 miles a couple of weeks ago and they are almost down to the warning indicators.
I've been pretty happy overall with the tires (although I wish had gotten more miles out of them), but I will probably replace them with the AT version of them as the HT's are not an aggressive tread pattern and are terrible on the ice and snow.
Although I think the HT's would make an excellent trailer tire.
If you are trailering a lot, the 40k would be good in my opinion, I have the 20 inch and it looks like I'm going well into the 50K plus range at my current usage, but I'm only trailering at 20% of the current miles on them. The 20 inch is a big tire with lots of rubber, so there may be something to the size also. I'm with you on the Transforce being a good tire for a rig. BTW, I have the ATs on my RAM 6.7 Crew, and they perform well on all surfaces. I'm not saying they are the best, I am saying though, I can't find anything to complain about with them. They are a good blend in highway and off road, and they have the weight rating I need.