Sun, blazing down on an unprotected tire will deteriorate it, especially the economy tires. That spare is your ticket to being able to get home without a hassle. Buy a new Carlisle trailer rated tire and get a tire cover. Periodicaly clean and dress the tire. Should last you many years.
It sounds like you have dry rot. Not using the tire and the UV in sunlight will beging to destoy some of the ingredients in the rubber and you'll beging to see exactly what your driving. Throw her away and get a new one and flollow the prevoious advice