I just bought some spares a couple minutes ago from Stoltzfus. They were $30 each.
There are two sizes. I measured mine last time I was at the coach so I could order them, and mine are the larger ones. I suspect with your coach being smaller and having the 205/75/R15's, you probably have the smaller one. I measured 68 1/4" x 12", and Stoltzfus said 68 x 12. If I recall, I measured from the very bottom lip that wraps around on the bottom side, so an overall length. I measured the height the same way, from the bottom lip to the very top in the center. It was more of a rough estimate though, because I knew the length was what counted.
Just to warn you, shipping won't be cheap.
Also, I would suggest you keep the old skirt(s). When no more are available, one with cracks around the screw holes would be a lifesaver if a blowout or stone took out one.
Also, a little tip for installing it, make sure to put a bunch of putty tape behind (just like the original was) and don't put the screws in too tight. I wouldn't even use a drill. The reason they crack there is because they tightened them too tight at the factory.
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