When our coach was new I had plastic film applied to our headlights. At 8 years old they show no sign of fogging over. It's the same film used on the front caps, and I assume there is some sort of UV protection built into it. I had a stone take out the lens of one of my little "driving lights" right off the bat, so the only reason I had the film put on the headlights was to help mitigate such damage to them.
The only issue is that due to the curvature of the lens, over time and seasons the film tends to buckle along a couple edges, allowing dirt to get trapped underneath. The installer across the street from BCS replaced them once a few years back, and they looked great again out of the gate, but within a couple years the edge buckling appeared again. Nevertheless, no rock chipped headlamps and no fogging of their lenses.
I know Bruce, that used to do painting at BCS, could expertly micro-trim front cap film to get edges laid flat again, and he showed me how when the coach was new. But you have to get after such things before much dirt gets underneath, so I haven't ever tried that fix on the headlamp film buckles. I may yet give it a shot. Or check in once more with the installer the next time I'm at BCS.
Joel