Sorry Mike, I really don't have any suggestions. I would have suggested All-Rite, but you already tried that. When I was at the Sunline Factory last year, I asked about fender skirts because each of my '94's had cracked ones. I was unable to find anything about them online. I was told by Jessie (asst. to Sue in Parts) that the older fender skirts were made by a company that had the factory burn and they just shut down. Also, all the molds were lost in that fire, so replacements weren't possible by another company. Sunline had a few single and tandem axle fender skirts left, but I think they were light gray. I do know there weren't any black or white ones. Hopefully they used another company for yours. When I tried to find replacements for mine, all I could find is white (no black), so chances are you can find replacements somewhere.
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