Have you checked your local dealer? Most dealers have taillights of many different styles, including the one you have. I have seen them at the dealer near me, and you can get the whole light and lens, or just the lens.
I believe your light should look something like this:
The whole light is $12.95, whereas the replacement lens is $7.95. These prices are directly off of the General RV Website.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR