Like everyone else, I'm so sorry to hear that Prince is having problems. I will pray for him (and you). I hope the tests eventually point to a particular problem that can then be addressed.
Prince doesn't eat lots of chicken jerky type treats, does he? Just a stab in the dark as lots of dogs have had similar symptoms to what you describe after eating lots of those. Daisy was in the vet for several days, had lots of testing, etc. a couple of years ago, and I believe it was a result of those jerky treats. The vet himself never did figure it out. I kind of figured it out after the fact, although I had my suspicions while it was going on.