What supervision did you have under the old steward? Did you just come in and get on with your work then go home?
Just as a balance on this, it is sometimes the case that ifnot properly supervised for long periods of time we become lax. We don't notice it, we don't admit it and we resent it if anyone else says it and it takes a new broom to pull us up by the boot straps and get us back on track.
If I were you I would ask for a comprehensive breakdown of your job, what the standards are and where they say you are falling below standard. Self monitor with the standard firmly fixed in your head and even ask the steward to check the areas he wants improving.
At the end of the day just because you have been there twenty years doesn't mean you are good, it could be that your supervision was poor.