Owning a trailer is great until you realize those tires age even when they aren't moving. Sun damage and dry rot mean you are looking at a fresh set every few years to keep your gear safe on the highway. It is a necessary evil that usually hits right when you want to be out in the dirt.
The best time to buy is late winter when shops clear out old inventory to make room for summer stock. If you time it right, you can grab a full set of four for about $750 and split that cost into monthly bites while the tires are already on your axles.
Keep in mind that the 0% interest period is a hard deadline. If you still have a balance after the 18 months, the standard variable APR will apply to the remaining amount, which makes those tires much more expensive than you planned.