One other thing they claim that ST tires get are heavier sidewalls that have UV inhibitor in them. This very well may be true; I cannot confirm nor deny.
That being said, my truck has Load Range E tires on it, it sees much higher loadings per tire than the trailer does, and they'll run as fast as I care to go just fine.
Some manufacturers make tires that are dual rated for both LT and ST. One example is Maxxis. I have their UE-168's on my triple axle Avion. The tires are marked both LT and ST. They actually do have three ply sidewalls. Most tires only have a single ply. They may be "rated" as an eight ply, or what have you, but they really only have one. Anyway, the Maxxis tire I'm using has very heavy sidewalls, is speed rated to around 100mph, and you can get them in Load Range D or E. I got D's because I wanted 235 series to match the diameter of the 700 R 15's I replaced. 225's could be had in Load Range E, but they weren't tall enough. I do not know if the 235's would clear on the Airstream's fenderwells or not. It'd be tight; the 235's are wider than the 700R15's were. Each one is rated to like 2050lbs and I have six of them, so I've got over 12,000lbs of tire capacity on a trailer that grosses at 9600. I'd call it safe
Of note: I've heard many guys recommend to not run D's or E's on an Airstream due to the stiffness of the sidewalls. Rather, they recommend C's. I can't comment on that, only to say that Avion has a much heavier frame than Airstream and they are not prone to the tongue/frame/sag/separation problems that the older Airstreams had. So you might want to investigate a bit before you get a set of the stiff tires. I think D's might be OK, but E's would probably be too stiff. As well, you must take into account the rims on your trailer. The older aluminum wheels might not be able to take the pressure that you can put into a D or E tire, and so if you put an E tire (for example) on a C rated rim and put the full 80psi in it, you may very well split the rim. That would not be good!
I run 60psi in my D's. I've pulled it about 1800 miles or so to date with this setup and no problems at all.