Shaun,
glad to see you back here- so very sorry to hear about Rasha.
I did have a similar experience with my first dog (cocker/basset mix). It wasn't even that he was coughing- his bark was funny. so took him to the vet- he diagnosed fluid/CHF. He said that Danny would need to be on heart meds for the rest of his life- I said that's no problem. this was late December. I had classes in another state so we went to our rented place in Jan and started school. things seemed fine until he didn't eat his morning biscuit when I left for school- big food hound so not eating was a real issue. so I took him to a vet in that area. she called me to say that there was so much fluid that they couldn't get a good xray. so they put him on something to reduce the fluid (in an IV)- he pretty much was there all day. so she was suggesting taking him to this heart specialist vet. then when I went to pick him up, she said the fluid had cleared some and part of the reason why they couldn't see his heart was that it wasn't in the right place- there was so much cancer in his chest that it had pushed his heart out of place and was ultimately the cause of the heart issues. anyway, the vet said no point in going to the heart specialist. if he had still been eating, I would have taken him home but as it was, I had to have him put to sleep about a month after finding out there was a problem. I know that is definitely not what you want to hear but it sounds to me like Rasha's issue is different. I certainly hope so and if the xrays didn't show any abnormalities other than the fluid, then things could be fine.
Granite's Indy hung on for a long time with meds after his diagnosis with CHF.
so I'm not sure if the ultrasound will help you or not. You should ask the vet about meds to reduce the fluid. I know that Danny got some but I don't remember what they were but they weren't terribly expensive - but this was in 2004.
anyway, sorry that the lab tests were inconclusive. I know that they can be.