Yeah, it hasn't "developed a coating," its LOST a coating. You can try one of the headlamp resto kits and you can probably get it pretty clear again, but the UV protection will be gone and it will deteriorate faster the 2nd time around. The anti-UV coatings in those kits help, but they're not as good as the original.
Isn't that one of the years of Freightliner that uses the exact same headlamp as a Mercedes M-class SUV? Might be able to get a replacement that's in better shape at a boneyard, but not likely. You'd have to hope for a garaged M-class that was taken out one day and wrecked in the side or rear end, then didn't spend too long in the sun at the junkyard. :-/