If it has a plastic lens that is just dulled and yellowed then sanding/buffing with a restoration kit does seem to work. I got a 3M 39008 kit for around $10 on Amazon a did the headlights on my Class A yesterday. They look really good now!
I had previously tried a sand and wipe on clear coat type kit which looked great at first but didn't last a year before looking old again so I can't recommend that type of kit.
Left: sanded & buffed & rinsed off - 4 step kit
Right: previous 2 step wipe-on kit yellowed & dull after less that a year
side by side compare.JPG