Time is the only known here, and most would say it is time to change them. You are the only one that knows how the tires have been cared for. If they have been in the sun a lot, twisted hard on pavement when backing, bounced over the curb, run low on air, or often towed over 65mph.
Replacing with new ST tires, just before a long trip is no guarantee. Even a new tire can be defective. I'd be more comfortable running new ones closer to home, until proven. The only time I ever blew a tire, was a new tire just replaced before a trip.
If $$$ are tight, the old tires well cared for, you aren't loaded to the max, and will be driving under 65mph, I would run the old tires, at least on the March trip.
You asked for opinions, that is mine.
Jerry