When I looked up possible causes for gastroparesis, nervous system disorders is on the list, so MS could very well be the culprit. I do hope you can find some relief from this as it sounds quite miserable.
It looks like there might be new treatments on the horizon. You might want to look into the last two on this page to see if they might be right for you and whether you can find a local doc who performs these procedures:
http://www.mayoclinic.com/health/gas...ents-and-drugs