First step is to find a vet that you like and trust. Shelters have limited funds and do the best that they can, however, I doubt that they did a thorough exam, labs, dental check, etc
So I would rule out medical causes for her condition first.
I would try a quality kibble like Zignature, if you think that it’s too expensive look at Purina Pro Plan Sensitive Salmon, add a bite of real food to it, like chopped broiled chicken liver (fattening and yummy) or even a little scrambled egg. Add a splash of water.
Feed twice a day….you didn’t mention her age…maybe 3 or 4 small meals per day would be better, instead.
It takes an adult dog a few months (maybe 6 months) to adjust and feel safe in a new home, give her some time. Best of luck
Ps: If you are receptive to science based veterinary medicine, you may find this site helpful
http://skeptvet.com/Blog/
This is not veterinary advice; consult your veterinarian.
