I have no experience with A-Frames specifically, but I believe the skylights in Forrest River products are all made of Acrylic (not Lexan). I know my Rockwood 5th Wheel's was Acrylic. I switched to an aftermarket Lexan skylight after my original cracked in a hail storm. Lexan isn't perfect either though...although it is essentially "crack proof" it is soft and scratches very easily.
As a side note...I went with a white Lexan skylight to block heat. It is awesome. It lets in 90% of the light vs the smoked original, but cut down the heat dramatically.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)