SladeW--Welcome to the Forum!!!
I have a 1972 Sovereign
and we've been using Goodyear Workhorse 7.00x15LT-D for about 3 years and have "gotten away with it!"--though I don't recommend it! At the timeI didn't know any better and that's what the previous owner had used. For what it's worth, the pictures of the trailer tires shown in my 1972 service manual appear identical to those Goodyear Workhorses--sidewalls, tread and all!
For many years, there really were no ST or special trailer tires. I suspect they're a relative latecomer to the scene, or just did not see wide usage, in the era of our vintage trailers.
I can remember when radials made their appearance on trailers in the early 70's and there was reluctance of some Airstreamers to use them--for many reasons, some of which are not really valid now.
I feel that your first priority should be an ST tire in order to take advantage of (hopefully) stiffer sidewalls, better UV resistance and tread design for trailer purposes. You should make sure that a local facility is familar with trailers and have them fit tested the first time you buy if you're switching brands or styles. Next choose the proper weight rating. Then decide based on the ride height and wheel-well opening for your coach, if you can use a radial or not. I felt that I could not. The rear overhang is important as well. Any reduction in ride height will affect give you more propensity to drag the rear when coming down inclines. Radials are said to smooth the ride somewhat and run cooler.
Also review the postings with regards to balancing. Do not allow the tire dealer to dissuade you from balancing! There are many posts which you'll find to confirm this.
When you have the tire dismounted, it's a good time to check and make sure that your wheels are of the proper type and weight rating for a trailer application. Mine, for instance, are metal and say something along the line of Rockwell F15x6JJ DOT-T 85psi Cap 2780lbs.