It seems like no one is touching this one so lets stir the pot with some speculation....since I am not a step expert in any way. We will assume that the chassis battery (which powers the steps typically) is fully charged and that the step ground connection and connector is clean and solid too.
You say that the steps are fully extended, so I assume you mean that the motor seems to continue running for a few seconds after the steps actually stop moving? I believe that when fully extended (assuming the steps are not binding somehow) the gear motor arm hits pin stops at the end of travel in either direction (in or out) which stalls the motor and causes operating current to jump at which point the motor stops. Likely there is some electronics inside the motor which senses the running and stall current etc. I assume this is not done by the step controller electronics since steps without controllers must also have some mechanism to detect end of travel (exposed end of travel switch contacts for that purpose would likely get plugged with road grime etc). So either the steps are “not” actually fully extending (close but not full – binding late in the travel) so the motor has not quite reached the stop pin so it keeps running a second or two longer before current rise shuts it off.......or ........perhaps the end portion of the motor gears may be slightly worn or damaged so the current rise is delayed slightly?
If you disconnect power to the steps (for your safety) and then disconnect the motor pivot arm from the stairs......can you manually prove that the steps are not binding anywhere in their full travel? There is a video (link attached) that shows how one fellow moves the portion of gears used in the motor for step operation.......maybe worth a try if the steps are not binding?