Don't know the brand (too damn hot to crawl beneath) but our old 86 Tioga class C MH has one for the coach door.
It works off the engine vacuum, when you open the door (solenoid valve) or stop the engine it extends, retracts when engine is started and door is closed. The motor that retracts & keeps step up is just a simple vacuum device quite similar to the one for the cruise control. It is very light weight and takes up little room beneath frame. Step is spring loaded to extend & lock past center of the pivot device, one old spring broke a few years ago, I just installed a screen door spring and it works fine.
Some of you guys are rather ingenious, I bet you all could come up with something similar in short order.