This could have many causes. I assume your Vet checked for any kidney, liver or pancreas problems in the blood work? If she thinks it is allergues, not sure why she gave antibiotics. Dud she suggest maybe trying Pepcid? Well, you could try getting a second opinion. If you do, get all the records from your Vet to take with you including all blood test results and x-Ray films. Those are your records.
What food are you feeding and how often?
You might try getting some Pro Plan food for sensitive stomachs. Transition over a few days mixed with what you feed now. Try feeding 3-4 smaller meals a day and one later in the evening. Call your Vet and ask if you can try the Pepcid and the appropriate dose.
This is no cure all but may help in the absence of anything else going on.
Good luck and please update.
I have a dog here who has terrible allergies and has had them for 3 years
NEVER has Harley vomited mucas,,,would it be possible to take him to
a different vet for a second opinion perhaps another vet could find out
what is going on?
Jan