Pleural effusion in my dog, there's a mass, can't determine malignant or not, fluid accumulating.?

I have a 12 year old mixed dog.She has lost 4 pounds since December 07,when I saw her breathing heavily I took her to Emergency,she was drained 1.4 lt of pleural effusion, the next day her regular vet took a sonogram picture(please provide e-mail address to send) This was Jan 20th, they put her on Lasix 25mg twice a day, next to her 5mg Enalapril (she was taking this supposedly for an enlarged heart, by the last vet opinion he didn't think that was necessary) and last week, Feb 8th when I saw her breathing still a bit heavy, took her to her new vet and they drained 700cc(after 3 weeks from first drainage), at this time he's put her on Prednasone 5 mg twice a day. She's eating better, but still has not gained enough weight. Who should I need to see? they insist the heart is looking great and say cancer (there is a mass) but all the exams say can't rule either one.I have her also on Transfer Factor Plus (read was great, vet obliged).Please help me save my dog.

You are doing everything you can. Sometimes things just can't be fixed. She is nearing the end of her life.
She might never gain weight. Cancers cause weight loss.
I am so very sorry your dog is so sick. I hope she has a lot more good days to spend with you. But if she starts having a lot of bad days, find the courage to say Goodbye. You love her so very much. Don't let her die by drowning in her own body fluids or by starving because a cancer is taking all her reserves. Prepare yourself to say goodbye when it is the best for her.
I am so very sorry she is so ill.