kwikee step malfuncuntion driving through rain

I think your manual is #888
Ours got to doing that. Dry weather but it was a loose crimp in the ground wire Doug mentions. Coach Builder used one large butt crimp connector to join four smaller wires, two from each end of the connector. Worked intermittent and the door started cycling Open/Close.
To make sure it doesn't have power, you need to find and unplug the Four-Wire Connector. That's the whole step. There's also a small Two-Wire Connector to the Motor, but if you're struggling with it, and the step goes to run, you'll be trapped before you can get out. Unplug the Four-Wire!

When you go to work on your step, BE SURE to disconnect the batteries (All of them if you are unsure which one feeds it) Steps are VERY dangerous to work on as they can start moving without warning as you fiddle with wires if there is a poor connection/short/ground. Please avoid getting a new nickname of "Nubs"!

You should be under warranty for your 2018 motorhome? I doubt the control box got wet inside. They have those controls very well sealed and I have never had a control box get water intrusion on a Kwikee step. What WILL cause your problem is the door magnet or Pin switch whichever you have, the brown wire (1) that goes from the Door switch to the control box. IF that connection is not weather tight and water/corrosion gets to it, That may complete the ground signal to open and close causing the step to extend and retract. This connection is underneath at the step wire connections and NOT inside a sealed water tight area. The step door magnet/pin switch is a normally OPEN, which means NO ground signal is sent to open the step. IF you get water/corrosion intrusion, that would complete the ground signal and the step will extend, then retract as the corrosion is not a complete ground circuit. Doug

Yes, water is getting into the harness. The best fix is rubber expansion tape. Not permanent and waterproof.