This method changes a regular mattress to fit the curved walls of an AS. First image is the final sewing of the fabric, picture shows curved mattress, temporary pins still inserted.
The second picture shows the fabric seams open and the spring exposed. Springs will be removed, fabric trimmed, then fabric sewn back together.
Unique Tools: Hog rings, hog ring pliers, seam ripper, curved needle, upholstery thread.
Mark desired shape with felt pen, top and bottom,
Rip seams for a few inches them continue by pulling with hand,
Using the marked line remove springs, or spring clamps,
Delete all spring that are fully outside the line,
Push fractional springs below line a re-clamp them,
Trim felt and hog ring to springs, the shape should now be what you planned for,
Sew the fabric back together, using pins to hold, curve needle and strong thread.
I will add installed picture next.