Hoo boy- is that a long story!
Tires have a life, and that's especially true of ST's, assuming that's what you've got on the fiver. They're designed differently than pickup/car tires- much more sensitive to what's called "duty cycle" than anything else. The worst thing in the world for a trailer tire is to be left standing idle for long periods of time.
There are tons of pages of information out there from various sources, and one thing they all have in common is that on a trailer tire, tread reading is not an indicator of life left. Time and duty cycles is what counts, and most recommend replacing at three to five years of age regardless of how they "look". Here's a link
to a page that explains how to read the date code on the tire sidewall.