The fifth wheel is a 28 foot Keystone Cougar. The original tires were 225-75R15, Load Range D. All four of them were bad when I got it home, and Keystone agreed to replace them all, including the spare. Discount tire sold me the Tow Master tires in load range E. The first set disintegrated due to weather in about 18 months. They were replaced with the same type. The ones on the ground appear to be okay, but the spare has "rapidly decompressed" two times now.
Every 30 days, I check the pressure in the tires and make sure it's maintained at 70-75 psi. I wipe them down with 303 Aerospace, and keep them covered from direct exposure to the sun.
So now, it appears that Discount is going to replace them with some alternate Chinese tires called "Carlisle".