As for 'can't get my black tank empty', are you depending upon the indicator lights to determine if empty? If so, those are notorious for lying to you. Crud (paper residue, etc.) often covers the sensors giving a false indication. You need to clean your black tank well to clear that. Often that is easier said than done, but there are several products to aid in that process.
However, getting an 'empty' reading is not necessary for winterizing. If you drain the tanks using the normal process, that will be adequate for winterizing. Once drained, I like to pour about about a gallon of RV antifreeze into both tanks to ensure the valves are protected and don't go dry.