I am worried about your boy and my main concern is also Fanconi. My Bella is only a year old and I have not dealt with the disease directly. (I pray I don't have to). All I can say is learn all you can about the syndrome and don't be afraid to ask questions. Is your Vet. open to suggestions about treatment or is he/she sort of "running the show"? Fanconi can be managed with proper care (if that is what this is). You need a Vet. who has experience with Basenjis or who is very open to learning about the specific needs of Basenjis. I hope you find out very soon what is making your boy so sick. The sooner the better for his treatment and subsequent health…
For the vision loss, I would get him in to see a veterinarian opthamalogist. An exam usually runs about $35-$45 and you will get a specialists opinion about what is going on.
You do not mention whether or not your vet was ever able to confirm a diagnosis of diabetes or fanconi. This would really concern me. If your dog has either one of these conditions they could be contributing to the seizures and other symptoms.
As far as I know, diabetes was ruled out, and I know my mom mentioned Fanconi and took our vet that handy dandy printout for vets, and he seems to think it's a tumor. He has had extensive bloodwork done, and his urine was tested at the vet, all coming out normal.
As for the vision, we haven't noticed a thing with it, and the vet said it could very well me the aftermath of the allergic reaction, and he might have been in a fog. I guess he had laid a treat down to the side of him, and it was obvious that he only found it by scent; he literally just didn't see it. So we will have that checked again after a couple days have gone by and he is feeling better.
I will check on the basenji vet idea…I know my mom loves this new vet we have, and she is really really picky about the care our boy is getting. So even though I don't have all the details quite straight (it changes daily!), I trust her judgement. Thanks all for the support!! 
Will post more again later, OH is begging me to come watch 24!
Seems like you are really trying to get to the root of the problem. I hope to see him feeling better soon, as I'm sure you are… Where is his tumor possibly located?
They think it is in the brain or pituitary gland…he is feeling better right now, sleeping through the night finally!!
That's what I suspected from the symptoms… I hope it is in the early stages. I will keep you in my thoughts and prayers.
God bless, Lis.
It sounds like Cushing's Disease.
Yes, it does sound like it may be. The possible tumor, the seizures, thirst, excessive urination, etc… How is it treated?
Most patients are treated with a medication called Lysodren or Mitotane. Anipryl (Deprenyl) and Ketoconazole (Nizoral) are two other medications that can be used to treat Cushing's. If Pongo has an adrenal tumor, the adrenal gland may be removed and/or treated with Lysodren.
http://vetmedicine.about.com/library/viewers/uccushings.htm
Viewer Viewpoint - Cushing's Disease in the dog
