I have my coach in for service for the very same thing. The Dynamax does not have a "magnetic" switch, it has copper push pins that go in and out when the door is closed or open triggering the stairs. I first checked the wiring to see if anything was loose like a ground under the coach. All where tight. Now they believe it is a loose wire at the switch itself that turns it on or off or it could be a short....funny, I've now seen 4 posts with people having the same problem.
I was sweating a bit when I passed a "wide load" going down the freeway crossing my fingers that the steps did not decide to extend at the WRONG time....