Our '87 has the same problem. You can purchase the step tread, but they are a little pricey. Out of Doors Mart has all of the pieces, nuts, bolts and washers.
Being frugal, I looked for an option to purchasing a new step tread. I took two heavy wire coat hangers and snipped off the "hook" a and placed them over the left and right hinge arms so that the wires fill in the voids left from the steps wearing. The lower step is now level.
I guess another option would be to have a someone do some aluminum welding / brazing and fill in the voids with a little bead of welding.
Carl, Elaine & Finn The Beagle
2006 30' Classic W Slide & Limited Package Katarina
2006 GMC 2500 HD 6.6 Turbo Diesel Crew Cab 8' Bed
NQ3U (was KB3UOM)