A not untypical story here, I'm sad to say. Obviously, the chicken nuggets is a concern, because it will have all kinds of bad things in it - even the chicken is probably the lowest grade you can buy. On a more positive note, has your vet tried Mirtzapine? This can have wondrous effects on inappetence, stress and other kd symptoms.
Also, if you know the blood numbers, let me have them - particularly for BUN, creatinine, phosphorus, RBC, potassium, calcium and amylase.