The excessive thirst could be indicative of diabetes. He should probably have bloodwork done to make sure there is nothing out of balance. Check for things like diabetes and hypothyroidism.
The excessive thirst and urination do sound like Diabetes – LOL, that's how my two kids with Diabetes were diagnosed -- and out of whack blood sugars will lead to seizure. The over-drinking will dilute the urine, so a blood test would probably be in order to check for Diabetes.
I'm not sure how it would play out in a dog, but I'd certainly ask about it.
Okay, talked to mom again and she tells me that the vet believes that he has diabetes insipidus, but that this is difficult to diagnose so he isn't positive. Also, his thyroid level was slightly elevated, but nothing out of the normal range apparently.
What we really need to know now is if there is anything we can do to help or stop the seizures from happening…is there anything we can give him that anyone knows of or has experience with? We have put him on a new spread out diet to accomodate for the blood sugar levels. Mom is going to ask this to the vet on Thursday as well, but thought I would check here too to see if anyone's doggie has gone through something similar. She is also going to ask for the bloodwork records so that we can see what they say first hand. Thanks again for reading guys, it really means a lot to me!! 
Also just read that seizures and muscle deterioration can be symptoms of Fanconi as well. Do you think that even if the vet doesn't give a finite diagnosis of Fanconi that we should start treatment for it?
If he does not have elevated Blood Sugar, I would have the vet run a blood gas test. Also, you can contact Dr Gonto, he has talked to many Fanconi dog owners and may have additional insight, I believe his email address is listed on the Fanconi Protocol
Thanks Ivoss, that's a big help…I'll be sure to email him about all this!!
Just thought I would give an update on my doggie's condition.
He had a full thyroid panel, which came back negative for anything out of the ordinary. So our vet has put him on phenobarbital to try to prevent the seizures. It totally knocks him out when we give it to him. He hasn't had a seizure yet, so fingers crossed.
Got a call from my dad last night who said that he thought Pongo was having an allergic reaction. He had been given some peanut butter treats and some cashews earlier (he's never been given nuts, why now, I don't know!) so we were thinking it was coming from that. Apparently he looked like he wanted out of his own skin. So they took him to the all night vet, who gave him a steroid injection and prescribed Valium if he didn't calm down. Luckily he calmed down and went right to sleep (although he had an accident in the bed :eek: ) and today our regular vet said he seemed to be over it, but not to give him nuts again!
Our vet did some more tests, and it now appears that he may also be losing some of his vision! 
Will this ever end in happiness?!! Poor guy, he is only 10…we have a perfectly healthy female who is 13!! It's so sad to see all this happening to our little guy.
There are a number of Basenji people in So. California… if you go to the Basenji Club of America website and then to breeder listings, you might be able to find someone close to you and they should be able to recommend a Vet....
Have you have blood work done? To me that is the biggest and most important thing to have done.. it will tell you lots of things... or rule out lots of things. Also, I am very surprised why your Vet has not taken the urine sample himself? They cath. the dog and take it directly from the bladder... only takes a few minutes.... have they tried that? The only other way that you can find out if it is Fanconi, if you are not able to get a reading, but like Andrea, I find that strange... is to have blood gases done...
By the way to all…. I do blood work on mine all the time... at least once when they are young pups to have a base line, then any time before any type of surgery, at least every two years till age 9 or 10 and then every year after that...
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
