So the doctors don't have any idea what she has?
The stomach pain could be refereed pain from her lungs or a symptom in itself. Does she have pain anywhere else? I'm sure she's had chest x-rays. Have they taken a lung biopsy? You said the tests on the fluid were normal? You also said she had a TB test that was negative? What about pneumonia? Did the x-ray show anything like that?
Does she have any heart problems? That can sometimes cause pulmonary edema (fluid in the lungs). Has her heart been tested? Pneumonia can also be a cause. There are other causes that are probably less likely: exposure to toxins, kidney disease (have her kidneys been tested?), adverse drug reactions, trauma, and high altitudes.
According to a very respected medical clinic website in the U.S. (which I cannot name because I'd be advertising), most of the time this can be cleared up if the underlying condition is found and treated. I'll message you with the web address so you can double check this stuff with her doctor.
You said she isn't eating. Is that due to lack of appetite or does eating make her sick? Have you tried giving her those liquid meals (not the weight loss ones, the ones for seniors)?
I'm sorry, but I don't know what DTV is and when I search for it, all I get are converter box results. Stupid FCC.
How many doctors has she seen? If you can't get any answers out of this one, maybe a second opinion would be best. Has she seen a lung specialist?
I really hope you find what's wrong with your mother. This must be so scary and frustrating that they can't figure it out. Do some research into the topic and don't be afraid to ask the doctor questions. Make them answer them. Don't let them give up.
Good luck. Please write back with any further developments.