There have been a few people ask about what size spare they can use. The easy answer is simply use the same type, size and Load Range as the tire you are replacing. BUT for some this may not be easy to do. Lets look at a few scenarios.
1. You have a trailer with ST type tires. Your only option is an LT type tire or wait for a few days. Now I don't have any personal experience with needing an ST tire in a rush. The one time I needed to replace an ST tire on my race car hauler, I already had a spare. I decided I could wait till I got home to buy a replacement. So my advice to trailer owners is to carry a spare. When you do I advise you either keep it in a storage bay or under the trailer or under a white vinyl cover to prevent the Sun's heat from damaging it. The spare doesn't even have to be a new tire. Just be sure it is inspected inside and out. I would also suggest you keep the inflation low (10 to 20 psi) and be sure you have a compressor available. However, if I simply had to continue on my trip and had no spare, I probably would have bought an LT that had a load capacity that was greater than the original ST. This would have been either larger in size and or higher in Load Range. While the axles would have looked strange with one tire being larger in OD, I felt that I could continue if I simply dropped my travel speed to the 50 - 55 mph range.
2. You have a Motorhome with same size front and rear tires that have significantly different levels of wear. This requires a bit more work. With duals on the rear it is critically important that you have each tire in the pair of duals, matched in size and load capacity. I covered how to match tire OC in dual position HERE and HERE. If you can't get a tire that allows you to have a "matched" pair, it might be possible to take the two fronts and use them as a pair and have different size tires on the front till you can get to a tire dealer.
3. Your motorhome has different size fronts and rears. This presents other problems, especially if the fronts are much larger. There is a critical dimension called "Dual Spacing" that needs to be considered. Learn more HERE. I suggest you spend a few minutes now, while you are calm, and review your tire sizes. You can learn the Dual Spacing for your tires by consulting the dimensional information on the web site for your brand tires. THIS post shows how you can measure and confirm your current spacing. Knowing what you have and what you need can answer the question of what you might be able to do in an emergency. You might discover that you need to review your situation with your tire dealer.
4. Another fly in the ointment involves your wheels. Many Class-C motorhomes have identical steel wheels in all positions but some Class-A motorhomes may have both steel and aluminum in the rear dual position and you may have to have tires dismounted and switched around as not all wheels can be used as both inner and outer in dual position.
Depending on your situation you may have no choice but to be towed to a truck stop where you can wait to have the correct tire delivered.
You may find that you need to review your options with your tire dealer as it would be better to know your options now rather than make an expensive and incorrect decision on the side of the road while under pressure.