Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am so sorry to hear about your girl's intermittent episodes of drooling, refusal to eat and lethargy.
Drooling goes along with nausea and a decreased appetite.
What specific medical therapy has she received so far?
I am glad to hear that her blood tests and radiographs have been clear so far. It makes sense that the next step to get a diagnosis would be endoscopy and collecting biopsies to try to determine why she doesn't want to eat.
I understand that those procedures are very expensive and not all families can afford that.
If she has had a lot of reflux then she may have esophageal burns and that can decrease her appetite and make her drool.
It is possible that treating her with sucralfate to coat and help heal ulcers may make her comfortable enough to eat while she heals.
I also recommend an acid reducers to try and settle her stomach in case this is related to nausea and stomach acid reflux. Either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle her stomach, hopefully enough to get her comfortable and eating. These are quite safe and can be used long term if necessary.
I would also start a bland diet mix of 1/3 boiled, lean hamburger (or boiled, minced, white, skinless chicken) and 2/3 plain, boiled, white rice. Add low salt beef or chicken broth to make it soupy and easy to swallow and get more fluids in to her. Give small meals several times a day.
I would also recommend asking your veterinarian to prescribe an appetite stimulant called mirtazapine.
If they believe she is still nauseous then an injectable anti-nausea drug called Cerenia (maropitant) with follow up pills could be used.
But if those things aren't enough to get her going then the next reasonable step is endoscopy.
Intermittent episodes of nausea and drooling is tricky, if she hadn't been running a fever I would highly recommend testing her for Addison's disease if that hasn't been done yet. This is a poorly functioning adrenal gland that causes intermittent vomiting or loss of appetite, and lethargy and a vague feeling of being unwell. Dogs with Addison's often seem to feel much better with fluid therapy.
An inexpensive thing to try would be to worm her with pyrantel. Although parasites don't commonly cause intermittent vomiting the stomach worm does, and it is very hard to find on fecal testing as it rarely passes eggs. It responds well to pyrantel.
A barium study would be appropriate to outline a foreign body, and the barium could also attach to ulcers on the esophagus and stomach lining, thus could also help diagnose ulcers. You could get the same information from endoscopy, and endoscopy is more sensitive and would allow biopsies to be collected too, although it is more expensive. If her barium series is normal then endoscopy would still be the next step.
Please let me know if you have any further questions.