I would be replacing all tyres if they are 10years old and your planning to head properly off the blacktop or at least the two on the axle.
Lost 2 tyres (catastrophic failures ) on a tandem axle boat trailer that were about 6 years old in the space of 6 months, they had great tread depth, pressures were fine the rubber was just old, replaced the rest of them with new after that.
While there is no used by date for tyres, 5 to 6 years is a number that is thrown about as a reliable useful life span. Beyond that the rubber degrades/perishes and becomes more prone to failure.
I would be happy to do a big trip with only one trailer spare and a good puncture repair kit provided the tyres were in good nick. I did my central Aus trip including some pretty gnarly tracks and roads last year with only a single spare.
Good tyres, adjust pressure and speed to conditions and you’ll be sweet👍