Finally chased down the answer. Getting to the bottom was a challenge because I ran into "customer service" people who could read from a script but had no idea why they were giving me the answer they were.
Even got to the engineers at Tire & Rim Association and based on their answers it was obvious they had not thought about ST type tires or the unique nature of RV application so acted a little surprised when I identified the engineering analysis that indicated that the current loads too high for many trailers and that the load capacity should be decreased by 15% or more.
Anyway here is the bottom line.
For Goodyear Marathon tires only:
There is a Tech bulletin
PSB#2011-13 that uses information from TRA (blind leading the unknowing in my opinion but they are the ones that have to stand behind their warranty).
The tech bulletin says that the normal max speed for Marathon ST type tires is 65 mph. This speed can be increased up to 75 only if
the inflation pressure is increased by 10 psi.
Goodyear does not support or condone operation above 75 at any time.
The trailer owner is responsible to confirm the rim is rated for the new inflation level.
While TRA indicates a further increase in speed might be achieved Goodyear has decided not to adopt this option.
The person I talked with at Tire Rack said they had already changed their web site to indicate this applied to Goodyear tires only and that they were attempting to learn if any other ST tire manufacturers were willing to make a similar change. As of Saturday Oct 19 they had not identified any other tire manufacturer willing to make such a change.