To clean up the dust created when sanding drywall mud, add vinegar to the cleaning water. Without the vinegar, the dust will just smear everywhere.
With this you really will be able to give the waste disposal a good clean. Make vinegar ice cubes and feed them down the disposal. After grinding, run cold water through. The grinding effect of the ice will sort out any stubborn items and it will remove the odours and smells as well.