I did mine a few months ago with slides from Ryadon (Drawer Slides Inc). $110 shipped. 2250 Series 250lb full extension 12", and 3300 Series 500lb full extension 22".
Like Roy said, I think one of mine was direct fit, and one of them required drilling a few holes to match.
5/32" cobalt drill bit with cutting/tapping fluid preferable for drilling out old rivets.
I thought I would reassemble mine with nuts/bolts rather than rivets, but realized that some of the locations have to be rivets because the top and bottom steps overlap and will interfere with anything larger than a rivet.
While I had mine off the coach, I sanded and repainted the steps. Knocked the rust off the underside and used some rust-converter spray to see if that would help keep them healthier for a few years.
On reassembly, I had a little issue getting both steps and slides to move smoothly and having everything aligned. This may have mostly been because my steps were bent, and I was having to straighten the steps as well as aligning the new rails. With enough tweaking they work great now.
There's probably better advice around, but that's how I survived getting mine done.
As a general reminder to me, the steps don't retract when the house battery is disconnected

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