Checking for power is pretty simple. Most, and I hope the OP's stairs does, have a light that comes on anytime the stairs are active. Craw under the coach and have someone open the door. If the light comes on you have power.
The thing about the stairs if you notice there are no mechanical switches that let the controller know if the stairs are at a limit either in or out. What the controller does is monitor the amps the motor is demanding. When the stairs hits the limit of movement, in or out, the motor effectively binds and current increases and the controller shuts the motor off. What can happen is if the stairs are binding or jammed somehow the controller will do what it does and shut the motor off.
Another problem could be a poor ground. A bad ground will limit the volts available but for the motor to do it's work with a lower voltage the amps required will increase. More amps and the controller shuts the motor down. Don't think that is your problem since the normal indications are erratic movement.
There's a bunch of You Tube videos you can watch to help work the issue. Just be very careful. If your fingers are in the wrong place when the stairs activate you'll have a story to tell the grand kids along with a possible trip seeking out medical attention.
2006 Hurricane 31D built on a 2006 Ford F53