Do you park under trees ? What kind ?
What waxes or other chemicals have you used ?
Have you applied things like wax, Armor-All or Rain-X on a very hot day or onto a hot surface that was in bright sunlight.
Does a sprinkler splash the vehicle ? If so, are any chemicals added to the sprinkler water ?
Does rain wash anything onto the vehicle (from a roof or trees) ?
Did you apply anything to that original cover that was replaced ? Did you do it in the hot sun ?
Is it possible that a neighbor is spraying lawn chemicals that somehow end up on your truck ? Or maybe you have a company putting something onto your lawn that's splashing onto your truck ?
Could any of these things be happening where you park your truck at work or church, etc. ?
I once had a problem when I used a mild cleaner (Windex) on the interior surface of my dashboard when it was very hot. Took the finish right off.