Are you sure it is your dog that is howling? Can you leave a recorder/webcam on to check? I ask because I had a neighbor make the same complaint about my first girl, and she NEVER made noise. The super even told the neighbor that he found it hard to believe because she didn't even lift her head on the couch when they came in the apartment.
If it is Sweet Potato, there are a lot of good threads here on Separation Anxiety and how to work with it. I know my second girl had a terrible case but it was quiet...she just ate the plaster off the walls, the wood furniture, etc. We did a lot of alone training, class training and I found that the DAP diffuser worked wonders for her.
Good luck.