Using the temp. reading of a TPMS to ascertain if your tires are overheating is pointless since are only reading the ambient air temp. and it won't tell you what is happening inside the tread. The thing to look for with a TPMS is a wheel/tire that is off relative to the others but the apparent above normal temp. could be from a failed wheel bearing or brakes and nothing to do with the tire.
Besides the already noted causes of tire failures - speed, under-inflation & overloading - high ambient air temp. (like in the southern US), impact damage and UV damage can also be contributing causes. Hitting potholes, speed bumps or cracks in concrete on the interstate at high speed can cause failure of the cords in the tire. I've snapped the cords on 3-4 car tires on the concrete-slabbed I-5 in northern California and I sure wouldn't want to do that on our ST tires.
Goodyear Marathon tires can be run at 75mph if, and only if, you run with an extra 10 psi in them to 75psi. ST tires are typically rated for 65mph so unless your tire manufacturer states you can run at higher than 65mph, stick to max. 65mph. 65 is a decent speed safety-wise and also a reasonable speed for fuel consumption and above that you're fighting wind resistance.
Internal heat and heat cycles are a killer on tires. 65mph should be considered the "redline" of your towing speed to minimize internal heat buildup. LOTS and LOTS of excellent info. from a tire engineer in this blog:
RVtiresafety.com
