Kevin, honestly, given the places he's been, I would consider treating Buddy for tick-borne disease. The tests don't pick up all of them, so you can spend big bucks on the tests and still get a false negative. So often our vets just start treatment with doxycycline if our dogs are symptomatic even if the tests are negative.
The reason I suggest this is that the TBDs can become chronic and they can do really funky things with blood counts--like reduce the platelet levels enough to cause bleeding. I know for sure this happens with anaplasmosis and ehrlichiosis and I wouldn't be surprised if it doesn't happen for some of the other TBDs that I'm not familiar with. TBDs can also cause neurological symptoms like those Buddy sometimes experiences. So it might be worth a talk with your vet about a course of doxycycline. It's not a terribly expensive drug, and if you can get a prescription from your vet you might be able to get it even cheaper online. Just make sure you work with a reputable online pharmacy (your vet may even have a suggestion).
I wish you all the best for Buddy. I know how stressful these symptoms can be and I hope you get some answers soon
Keep us posted on how he's doing, k?