Thanks to everyone for all their good thoughts and advice. I really would be COMPLETELY in the dark about Fanconi if not for this forum, and at the mercy of these vets that know NOTHING about basenjis or fanconi. I was an idiot and didn't test her regularly, and I've lucked out in that it seems that her onset of Fanconi allowed a UTI to develop. If not for that UTI, and her peeing in the house right in front of my face, I would not have figured this out so soon. She peed on the rug in the living room right in front of me… and two days later, she peed on the couch where I normally sit. It's almost like she was warning me herself that something was going on that I should be paying closer attention to...
Dr. Gonto did tell me that by the time it gets to the point that the dog is drinking and urinating more (non UTI related), he/she could have been spilling sugar for up to 6 months already!
The worst part about this experience was finding a vet that had the capability of doing the blood gasses. Of course it had to be a weekend... and looking back I got a lot of mis-information from the people answering the phone. I confirmed a few times with the vet office that they could do the blood gasses onsite, and even then, the first thing the vet said to me yesterday after I got into the exam room was "you're not going to be happy, we don't have the cartridge to read the blood gasses." Ugh, I told her that I called and confirmed a few times and she said "weren't you the one that called and we suggested you go to Tufts or Angell?" and i said "yes, but that is 2 hours away and since then i've spoken to several members of the staff here who confirmed that you did have the machine and I clarified that it was not an arterial blood draw, but a venous blood draw that was needed." ACK. Turns out, even after all that clarification, she didn't realize what values we needed from the blood gasses and they did have the required cartridge. And she was trying to convince me that Tayda probably just had a UTI, and I said that sugar in urine is not normal for a UTI, and she told me that it IS possible...
Under less critical circumstances, I would have probably walked out of the office, but I just needed to get those blood gas numbers in my hands and into an email to Dr. Gonto....
Again, thanks to everyone for the support, this community is so awesome!