Vetrinary help for esa doggo needed

I can't believe there wasn't a single Pet forum board on here. You'd think there'd be one for ESA animals and what not because they seem pretty important to me, so I apologize if this is the wrong forum but I don't know where else to post.

That being said, I need the help of any vet who is a member on here. I have a 7 year old Chihuahua-Jack Russell mix that is about 18.9 pounds, and ever since the new puppy came home, she has started isolating.

Well, now, her symptoms have expanded to include lethargy, occasional panting, loss of appetite, uninterested in treats or even water, but doesn't seem in pain. She doesn't wince, her eyes are clear and lucid. She has diarrhea, and her stool is dark brown and occasionally a little more to the reddish with spotting. She's been to the vet twice, first time to give her an anti-nausea injection and anti-diarrhea med, but she's not eating so that's not really helpful. Second time they gave her light sedation (because she gets a little anxiety-prone at the vet), subcutaneous fluids, and an anti-nausea and anti-diarrhea injection. Metronidazole was one of them. That was yesterday. The sedation has worn off and 24 hours later, Juno shows no change. The vet here (Star of Texas in Austin) is saying that they think it's some kind of viral gastroenteritis. The new puppy came down with Parvovirus, but the vet literally caught it during its incubation phase where the puppy was just starting to be symptomatic, and we treated him at home, and he's completely recovered. Juno was vaccinated against Parvo, and was tested for it, but it was negative. Today, she just started coughing.

I don't have the money to pay for all the testing because there's a dozen things it could be. I am already living off my credit card because I lost my job on my day off, and I am already teetering on bankruptcy. Juno is my ESA dog, so bankruptcy is not entirely out of the question as far as I'm concerned. I am impulsive enough to say eff it and go whistling into abject poverty and endless bankruptcy court for her.

Could it be something as simple as depression? I don't know. I obsess over everything so I am already drawing highly-questionable conclusions about her symptoms based on my extraordinarily limited knowledge, which is why I need expert help so I can narrow it down to the most likely suspect.

Please help me. I cannot lose my doggo. My life is a house of cards. Juno is a structural piece of that. I love my vets, but I am panicking and need a bit more in-depth help. I am hoping someone on here is a vet willing to point me in the right direction. I know a CBC is probably in order ($200) and probably X-rays ($175), and they'll probably give her x medication ($100-200?), as well as a stool sample ($75) since they only tested for Parvo. I don't know how they check for heart worms (even though she's on meds for it) which means ($$$?).

I'm drowning right now. and I cannot lost my best friend in the process.