The time has come (probably passed) to replace the tires on the trailer.
Currently we have Carlisle ST205/75D14's mounted.
We bought the boat in 2012, trailer came with these tires and they have been OK.

There was no spare tire on the trailer, so I purchased a WD H188ST w/rim at Walmart.
In retrospect, I see that appears to be a crappy tire. but may be OK as a spare. (internet reviews)

The question is: What are decent tire brands?

Most of our trips are about 120mi round trip (60mi to the coast and back)
Combination highway and well maintained roads.

There are no domestically produced ST tires, with the possible exception of Goodyear Marathon which regardless of where it is built is a horrible product well known for tread separations and beating the shit out of the trailer like a big weed eater.

What you have now is diagonal ply, and pretty poor quality at that.

Take your pick of radial ST tires made in China and keep them aired up tight, and they will be an improvement over what you have.

Towmax is pretty widely available and is about as good as chinaman ST tires go.