I'm so very sorry for your loss and troubles 
Please do look into sending the body yourself to your TN state lab. I could be wrong, but pricing for diagnostics in TN is extremely reasonable. $500 is too much, imho. Ask the lab about getting a shipping label as well. I hope you find some answers.
https://www.tn.gov/agriculture/consumers/pets/animal-health-diagnostic-lab0.html
Now on another thought. Have you checked your feed to make sure it's not moldy? Nothing has gotten wet and the feed is fresh?
Please do look into sending the body yourself to your TN state lab. I could be wrong, but pricing for diagnostics in TN is extremely reasonable. $500 is too much, imho. Ask the lab about getting a shipping label as well. I hope you find some answers.
https://www.tn.gov/agriculture/consumers/pets/animal-health-diagnostic-lab0.html
Now on another thought. Have you checked your feed to make sure it's not moldy? Nothing has gotten wet and the feed is fresh?
