I am sorry you are not getting any answers from your doctor. I know it is so frustrating.
I googled ammonia smell urine and here is something I found:
Has she been checked for a urinary tract infection?
Ok, what comes to mind here is bacteria in the urine. I know you said you do not have typical UTI symptoms, but from what I have researched the smell of ammonia in the urine comes from "urea". Bacteria that contaminate the urine will convert chemicals in the urine into smelling chemicals that are responsible for the distinctive smell, such as ammonia.
There are foods that can add to the excretion of nitrogen compounds (ammonia and urea). These foods would be red meats, or high protein foods.