I think this falls under the category of "They don't build them like they used to".
Mine started doing the same sagging and flexing after just a couple of months. Some of those giant "rivets" developed slop and one was pulled almost all the way through.
On another forum someone else complained about the same thing and we compared pictures. His step was failing in the same places and the same way mine was.
A Kwikee steps program manager announced himself on an FB forum so I posted my pictures and he asked for more details. I provided him the info and PM'd him the part number and serial number. The result was yet one more factory person vanishing.
I did get a new set installed under warranty that were far more solid. I still didn't like how much they moved so I started putting leveling blocks under the stairs to make them solid. That got old so I bought a pair of Camco 44531 step stabilizers:
I wish they would fall over if the steps get retracted inadvertently, though. The leveling blocks would just get slid off of each other.
The motorhome front tires are currently on 8" of ramp due to the slope of the driveway, which is the reason for the two 2x12 blocks. I'll probably still use one just to spread the weight out when on unpaved surfaces. The steps are as solid as the ones in a house now.
In the one picture you can see the air between the step support and the step because the rivet is pulling through. The other pictures shows how much of that rivet has pulled through compared to another.
I, too, bought some bolts and self-locking nuts in case the steps failed before the replacement assembly could get installed.