Well, now that the more experienced folk have confirmed it is a case of clear coat failure, I'll tell you what I did (as I did not have the money to repaint my truck either):
My truck was a charcoal grey color that had clear coat failure on the roof before I bought it and, instead of multiple areas of failure as on your truck, mine was just a very big area-- shaped like a sunroof and in the same general area as a sunroof, but smaller in size than a real sunroof.
My solution did not improve the area, but it did prevent it from getting worse, and I had the truck for about five or six years. The truck was a daily driver and was always parked outside.
All I did was wash the truck with a nice car soap (I use Meguiars) and then followed it up with a coat of Turtle Wax. The truck was not washed on a consistent schedule, but varied anywhere from 1-3 times/month. I would apply the wax at least once a month, or whenever it "looked like" it needed it.
Again, nothing fancy, but the areas of failure did not get any larger or any worse with this treatment. The paint on your truck may be different, but play around with some products and see if you can prevent the area from expanding...
Take care.
