My Poor Jazz is Sick!!

Jazz, our 9 year old beagle has been unwell on and off over the last few months. The most recent bout lasting for more than two weeks now. We have taken her to two vets on 6 different occasions and she has been on various medications but nothing is making her better and neither vet has a clue as to what is making her sick. Anyone who knows beagles knows they eat anything and everything but for the first time in her life Jazz was not eating anything for about 3 days and that's what really set the alarm bells ringing. Now she will only eat chicken breasts and soft chicken dog food very slowly but none of her usual foods. She is really struggling to get up and down the stairs, has diarrhea and  is peeing a lot. Her breathing is very rapid and after drinking or eating she coughs. She is just lying around and not her usual active self at all. I am very worried about her and wondered if anyone out there might have had similar problems with their dogs or might know what is wrong with her.