Hi Sasha's Dad!
I can see why you are leery of what your vets have been saying since they are all over the place on your dog's diagnosis!!!
If you read most of the other posts on the board, the vets take too long to decide that it is TCC assuming (rightly so) that the problem is a UTI.
So first off, I would try to relax and assume the best. (I tell you this from experience as my only regret since my pup was diagnosed was how much time I spent worrying when I should have been enjoying every second with my dog!!!) Next, I would wait and see if the antibiotics resolve her symptoms.
Although you wouldn't think it to read this board, TCC is not really that common a cancer, although it is the most common bladder cancer. I would also be more inclined to think that your pup's weight loss (and 5 pounds on how big a dog? My golden could and should probably drop 5 pounds, but that's not going to happen!) is more from Sasha's other problems. Usually, most dogs with TCC have no symptoms --other than their urinary difficulties-- until it is very advanced. Loss of appetite is not one you hear about.
Thickening of the bladder wall is definitely a symptom of a UTI as well as TCC. If you own vet does not feel he is able to read the first ultrasound, can he not give you a referral to someone who can?
By the time you read this, I hope Sasha is doing much better and you are feeling better as well.
Good luck,
Marty