Since no one has answered yet, I'll post some ideas (but I have manual steps so don't know).
I looked through a bunch of years' manuals, and they never show anything about the steps being on the Chinook breaker panel in the cab by your left shin, nor do they show a position for it in the "brown box" house fuse area. So my guess is there is an inline fuse ... somewhere. I know,
I feel like maybe I read about this at some point, but since I don't have electric steps I don't remember the details. I know, not that helpful. I guess if no-one pipes in, you could start checking with a meter.
I will say that on my Concourse, there is a bundle of wires that goes from the taillight area, up along the driver's side frame rail (in black loom), through the driver's step well (that black plastic liner is easily removed and then you can see it all). But that is for the tail/running lights, which terminate at the Chinook block by driver's left shin.
But the steps have to do with the ignition... Obviously if someone knows, so much the easier. But otherwise, I would look to see where the step motor is (under the steps?) and then try to trace the wires. Options I can think of are with that bundle along the frame rail, or with the generator driver's overhead start wires which run basically from the passenger taillight area, up to near the roof, and then up in the wire chase on the passenger side in the upper cabinets, then into the roof area (for the over-driver's-shoulder switches).
Another potentially useless factoid: My Chinook came with electric dump valves (now there is a solution looking for a problem!), and those wires went up and had something to do with the ignition as well (they would only activate with ignition on). However, I never traced them the whole way. And they weren't on any specified fuse panel -- so probably a similar inline fuse somewhere?
I know, that's not much to go on.
BG