Yes. I must buy a cover for that spare tyre. These days, the boat does not get a lot of use and so I would not be bothered rotating the wheels because the tread will never wear out in my lifetime but a cover for that spare makes great sense.
Many years ago, I had been walking across a bridge across a high sided river and which had no side guards nor footpath since it was well out in the bush where you would not normally find pedestrians. I and my work colleague had just stepped off the bridge when a fast moving car towing a caravan came along and just as he reached that bridge, a tyre on the caravan burst and the car and caravan did a 360º spin right where we had been standing. My older colleague fell apart, physically shaking and muttering repeatedly, “That could have killed us! We should be dead!” He was very shook up because a minute earlier, when we were on the bridge , there was nowhere for us to escape to other than jumping off into a very long fall onto rocky river banks. Clearly trailer tyres are just as important as car tyres.
Use light truck tyres on your trailer, not sedan road radials especially if you are carting gravel. Your local tyre merchant should have told you this!
I think the point in context was if using tyres of doubtful condition they are most likely to fail when you most need them, IE have the trailer loaded to it’s maximum load.
It’s easy to check and follow the recommendation of the manufacturer for tyre size and rating. It is found from the VIN ‘Compliance Plate’ details attached to any modern registered trailer.
Whether a 6x4 box trailer might benefit from upgrading to LT rated tyres is a further discussion. Correctly inflated for the load I’ve had no issues with the specified sedan radials on Ford Falcon standard 15” rims. Perhaps there are also those who overlook that need before picking up at the landscape supplier.