Did they do a HIDDA scan? That can show things ultrasound cannot. What about an MRCP? Instead of an invasive endoscope-type procedure, it is done via MRI and looks at the gallbladder, common bile duct, etc. From others who have had what you do, maybe your gallbladder is inflamed without stones or sludge is passing through - that is not seen on the US or any other type of scan.
In this current medicine era, hospitals will keep you under observation status or full admission. You're right - if you are not dying they may not keep you. Another thing to keep in mind - just because some docs can't see something - doesn't mean they are right. Years ago, a doc told me that if docs can't find answers they can tend to blame it on the patient.
Double check with your insurance, too. There are probably certain things they may not cover, such as a longer hospital stay without anything major going on.