This reply and others like it from other tire manufacturers of ST tires is why I stopped raging on just Marathons. Its a tire type/class problem. Again, would I put ST tires on my boat trailer that goes across town at 45 miles an hour or less on a trip that lasts 45 minutes or less? Yes I would.
That being said, Cooper makes a Custom Trailer Plus, ST22575R15
Cooper Tires - Tire Specifications
with a speed rating of S (112 MPH).
Tire Tech - Speed Rating, Load Index & Service Descriptions
These Cooper Sustom Trailer Plus Tires are hard to get a hold of, but if they perform as advertised, theoritically, you will not have any trouble with them.
BF Goodrich Commercial T/As in the LT22575R16 Load Range D are a lot more available on the road (you still hit stuff on the road you know).