I bought a new tire for the trailer.
When we bought the trailer, the seller assured me that two new tires had been installed under the trailer. They looked decent, so I accepted the statement at face value, and didn't think about it.
Fast forward to two weekends ago. We very nearly suffered a blowout on one of the tires. We only put about 5000 - 6000 kms on those tires, which seems like very little. So I started digging. Turns out the tires that the previous owner bought and installed were 5.30x12 (the correct size), but load rating B, good for only about 850lbs. My trailer has a GVWR of 2300lbs, and when travelling I load it up with firewood, food, water, bikes, and the canoe. So I'm probably pretty close to that max GVWR. That is too much weight for Load Range B tires. It goes a long way towards explaining why those tires didn't last long.
So anyway, when I found the faulty tire, I replaced it with the spare tire, which I think is original to the trailer. It was in new condition, and was load range C (1050lbs). So I bought another 5.30x12C tire from Canadian Tire for $60. Canadian Tire wanted $20 to mount it. Another garage wanted $30, and fountain tire wanted $15. Funnily enough, I took it to my Toyota Dealership next door to Canadian Tire, and they did it right away for $13.50.
So, I replaced the other load range B with the new tire, and relegated the Load Range B tire to the back of the trailer to act as our spare. I now have two correctly rated tires as the main tires on my trailer.