Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very concerned about your older girl Mia's unsteadiness on her feet, lethargy, complete lack of appetite and refusal to drink much water.
She likely isn't passing stool or much urine as there isn't much to pass if she isn't eating or drinking anything.
Her unsteadiness and lethargy may be due to low blood sugar from not eating, but at her age is more likely due to metabolic toxins building up in her body due to organ failure or some sort of cancer.
I highly recommend that she see a veterinarian as soon as possible. I understand that might not be the answer that you were hoping for, but she sounds much sicker than home care treatment can help.
Lack of an appetite and lethargy can be related to eating something she should not have, too many treats or table food, eating foreign material (which could cause a full or partial gastrointestinal obstruction), chronic pancreatitis, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, Addison's disease (hypoadrenocorticism), inflammatory bowel disease, or even cancer.
It sounds like her appetite has been slowly waning for a while. Although you kept her eating something by adding things to her food that incentive didn't last for long, so it is time to get her help.
If you cannot have her examined soon for whatever reason there are some things you can try at home.
At home to try and settle her stomach and get her eating you can give either:
1) Pepcid ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours
These are acid reducers and may help her feel less nauseous so that she will eat and drink. They are quite safe and can be used for several days if necessary. You can crush the pill, mix with a little broth, and aspirate the mixture into a dosing syringe or medicine dropper. Then place the tip of the dropper into her mouth at the corner where her upper and lower lips meet, guiding the tip between her molars/premolars while holding her mouth closed and tipping her head up slightly to make her swallow.
You can also do this to force in broth and water every hour. Try to give 2-3 tablespoons per hour.
A couple of hours after giving the acid reducer I recommend offering meat baby foods or a bland diet of 1/3 boiled, minced, white skinless chicken or boiled, lean hamburger and 2/3 boiled white rice mixed with some low salt chicken or beef broth to make it easy to lap up and swallow.
If she continues to not eat she should see her veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.