That looks quite nicely done.
My motorhome has something very similar from the factory, with a little light installed on the dashboard that comes on if the step is extended when the ignition is on. They used a microswitch mounted to the step motor that the operating arm activates. If you have a power step, this may be a simpler mounting point for the switch as there's much less play in the mechanism there. Of course, if it's a manual step, there's no motor, so it's not an option.
I kind of like the audible alarm, as the dashboard light is sometimes too easy to ignore, particularly as it happens to often be obscured by a steering wheel spoke on my RV.
(My steps, while motorized, are not automatic; there's just a switch like a power window switch at the entryway to make them wind in or out.)